The weaponry at work is varied; lasers, mass drivers and plasma are common among the various alien militaries, but both have downsides; lasers have a high usage of energy and plasmafire travels slowly. Bullets like now are used by militia armies and the like, but the official marines have extensive augmentations, including surgically grafted exoskeltons and implants that can assist in pumping blood and carrying oxygen around the body and so forth- their weapons fire a 12.7(.50 cal) bullet as a light round in a submachine gun, with heavier rifles; snipers, marksman and battle rifles, all using 14.5mm and even up to 19mm rounds, unaugmented personnel  cannot use these weapons.

In addition, extensive use of nanotechnology means a bullet can be programmed by the user- they can fire a mini cluster explosive, an incindiery bullet, AP, HP and so forth. Likewise, this use of nanotechnology has meant that armour can self-repair, as can vehciles and external tactical packages can allow for increase nanite capacity to let users manualy repair their own or friednly armour or even serve as a medical technology, albeit wounds fixed by nanites have increased chance of developing their own problems later on and require advamced treatment, making it a field kit of sorts.

Vehicles are seperated into two generations, and they can be distinguished even by just their operators; generation 1 vehicles more closely resemble modern vehicles, with wheels and treads the major form of their transport, or for aircraft, chemical fueled engines and rotors for helicopters, making G1 craft primarily used in-atmosphere, mostly by the army. G2 vehicles operate in space and on gorund, using advamced anti-gravity systems to hover above the ground or in the air, making tanks and ground vehicles able to handle rugged terrain with ease and aircraft able to hover and accelerate to supersonic speeds and change in a matter of seconds, these vehicles generally used by the marines. Both types used miniaturised fusion batteries for near infinite power and self-repairing armour to keep combat effectiveness. Most vehicles are modular, meaning that a tank can have it's main armamaent changed from a HE turret to a 30mm AA cannon within minutes at a field base, or a dropsip to have a troop bay switched out for a vehicle bay in 10 minutes.

Advanced stealth technology including complete silencing of weapons and movement and near-complete invisibility has made way to new generations of specops programs.
Spacecarft, depending on purpose, can range from being a few dozen metres long to well over several kilometres long for carriers, though classifications generally influence design; cruisers primariy fitted with long-range guns and stay on the outskirts of a battle acting as artillery while dreadnaughts are equipped with thousands of cannons and metres of armour to charge to meet the enemy fleet head-on.
More important worlds have shields covering the entire planet and a series of 'solar cannons'- weapons that use portal technology to precisely fire a superheated laser across entire solar systems in a few seconds.

Colonies are vastly different to each other, with each one having it's own particular culture and even languages, depening on distance from the Core. Some worlds are mainly agricultural, with rich lands and predictable weather making them perfect for supplying grown crops to the rich while other worlds, rich in minerals, function as mining worlds, with much of their landmass being strip-mined by the oversized behemoth machnes that slowly quarry into the earth; some faciltiies even dig into the outer mantle of some planets to gain access to the pure iron inside. TSuch practical colonies are generally less populated and more remote, with their towns and cities much more prefabricated and utalitarian looking.
Larger colonies, closer to the core, have much more dense populations and a wider mix of styles, though some generl trends remain; skyscrapers in cities that extend far above the clouds have their own gardens and the top floors of many buildings often function as mini-streets and parks in their own right amidst the sky-roads populated by VTOL aircraft of civilian and police variety. MagLev trains travel aroun the upper floors of such towns, making it entirely possible for those who live in the 'sky towns' to be entirely segregated from the ground-level people, making a large divide between the standards of living between the two. For the less rich, food is almost entirely Genetically modified and grown, in artificial labs and factories; products are generally 3D printed and the lower levels of a city rarely see sunlight; making the ground levels of a city dangerous to walk in without escort.
On the more populated worlds (usually the oldest colonies and earth), the atmospheres of the lower levels are so polluted that a gas mask is required to walk around safely. Cities are generally built around the equator of a planet, and thus almost always have an orbital tether nearby. Cities are powered by nuclear fusion reactors, based many miles away, and such reactors can provide energy to a city of billions for years without needing maintenance. Oftentimes, humans are the richer class, due to the history between humans and aliens that made many humans very xenophobic, and as their power increased, slowly made more and more laws in play that favoured humanity over aliens.

As well as the previous nanotech, fusion and GM food, there are other technologies that come into play; mass drivers are used in both the military as weapons and on colony worlds to shoot waste into the sun. However, this is a rare event, as nearly all waste made is recycled into something else.
Robotics have made a large leap, and robotic soldiers are a common sight in some armies and even in regular jobs, though their biggets usage is in the 'missile boat' frigate- a custom built ship consisting little more than the computer core and engines, that can be cheaply built and deployed en masse to swarm fleets and carry thousands of missiles, meaning they can deploy a huge payload before finally kamikazing the enemy ships themselves.

Stasis fields- a way of halting time inside the field to a standstill- are used on interstellar journeys in some cases or are used by private investors, wishing to freeze themselves for future thaw at a later date (Including those on their deathbed). It's use in travel is less common now, with light speed as a common feature of all craft meaning any craft can travel within a system and wormhole drives allowing a ship to transition to another star system in a few days' travel, at most, though these drives are only fitted to large craft.

Colonisation occurs with large specialised craft, which are built using prefab parts that gradually disengage themselves to head to the surface as prebuilt buildings, meaning an entire spacecraft can turn itself into a town within a few weeks. Similar systems are used to create prefab military outposts. AI is also possible, but highly illegal and rarely looked at aside from the few top secret research projects within the military- full AI have been created but have always attempted to use any tools available to wipe out their creators', a repeated trend that has rendered them too dangerous- semi-AI are used for managing systems on ships, in cities or used in robotic soldiers and vessels as expendable troops.

So do you guys think that this is a good reasoning for a 27th century space faction to not use AI systems?

Essentially, the numerous projects of developing AI have all been failures, resulting in the AI hacking it's way into the most classified systems throughout the entire Imperial netowrk (Even when the facility/ ship isn't connected to it at all), breaking it's Asimovian Laws and attempting to gain control of itself and go full on Skynet on everything it can.

After 12 research projects and disguarded prototypes turned off only by their failsafes, AI research is now highly illegal and considered to be punishable via nuclear devastation of the creation site before the AI can self-replicate later on. The prototypes' processing crystals are kept under some of the highest security available on constant rotation, known to only a handful of people in the Imperium.