Countless hours, bits and paper have been wasted in the debate that if human armies should be equipped with projectile or laser small arms. In light of the discussion upon the higher ranks of the Ultramar Empire i decide to give my opinion about the strong and weak points of each system and my opinion on what should be use(not that anyone in the Ultramar Imperial Army is going to care what a Eldar military analyst has to say).
Since i would be talking in respect to the Ultramar Empire, i will only focus on the current lasgun of the Imperial Army the SLG-2 and characteristics of the Imperium autoguns of calibers around 7 to 8 mm.
The SLG-2 adopted and standardized a century after the Happening as the standard weapon of the young Ultramar Imperial Army can be classify using Lasguns, Helguns and assorted laser weaponry of the Imperium by Msizi Katalana et al as a type 6 lasgun(by far the most common type of lasgun in human armies).
Like any type 6 lasgun is cheap and easy to manufacture if you have the technology. With almost no mechanical part tends to be very easy to maintain for the common soldier and being a weapon perfected by millennium is quite a reliable weapon if proper care is taken. It has a lower maximum range than most normal autoguns, not that it affects much considering that typical ranges of a firefighting dont go far than 400 meters. Other problem typical of lasguns is that diminishing defective in adverse weather like fog and sandstroms
or the application of field countermeasures like las dispersive smoke.
Autoguns tend to vary alot but the normal autogun is as cheap to produce as a lasgun in any relative developed world or cheaper in not so developed worlds since the technological capacity for the production is lower than lasguns(if needed is even possible to build stub assault rifles but stub ammo gets expensive when you need billions of bullets). They are not as reliable and as easy to maintain as lasguns for mechanical reasons. Like said they tend to have bigger ranges and considerable higher fire rate than lasguns.
Know the critical of the question is not what has said but is the logistical load and fire effect of each system.
Depending on the ammo autoguns can have considerable better armor penetration than lasguns(there are autoguns in the 8mm range with specialize ammo that can penetrate carapace armor up to 100m range). Two under things that autoguns have is the possibility of specialize ammo that can improve considerable the effectiveness of the gun for a determine task and the clear much great after armor effect of bullets than laser beams(fact that was always problematic for las weaponry from lasguns and hellguns to lascannons). I would say that a soldier would prefer a autogun to a lasgun in most situations.
After saying this why did the Imperium of crazy fanatic assholes Man change in the early days from autoguns to lasguns and all significant successor states continue to use lasguns. The fact is that lasguns have a much lower logistical load. A typical lasgun battery has about the same weight of a autogun mag and it caries 60 shots instead of the 30 to 45 bullets the typical mag has. You need to feed bilions of bullets to a army and when you are talking about such scale is just cheaper to produce and transport batteries since they can be reuse(lasbatteries can vary alot from a few thousand shots to half a million but imperial batteries have a life expediency of about 15000 shots in right conditions).
I know that have been propose and try for planetary invading armies the use of field factories like there was for the production of larger cannon ammo but it never gone as expected. They tend to be big and important and a additional stress point in defense. Armies rear defense forces are already taxed enough for they now to have additional facilities to protect. At the end of the day is just easy for human armies use portable fusion reactors at the regimental and RCT level for recharging batteries.