It's certainly nasty, but a few drawbacks:
It uses Gunnery as its skill (as opposed to Ranged(Heavy))
Encumbrance of 9
Costs 6000 credits AND is restricted
And the big one: Cumbersome 5 quality. Which means if your Brawn is not 5 (or higher), you're automatically upgrading the difficulty a number of times equal to 5 minus your Brawn.
I'd argue the fact it uses gunnery is a bonus not a drawback, the fewer skills you need to buy the better and the fact you use the same skill vehicle weapons is a plus.
Encumbrance is a pain but it is THE big gun, you could probably mitigate it with gear.
Cumbersome is actually less of an issue than encumbrance as you can bring it down to 3 with a weapon harness.
Not to mention it specifically states in the description that i comes with a tripod, that 3 ACTIONS are REQUIRED to set it up, usually by a team of 3 personal, and lastly if your not an Imperial soldier and are caught with it they will put you in an Imperial Prison for YEARS.
Don't use the tripod. The HoB says it can be used without it although non Heavy characters will likely struggle.
Please this is EoE pretty much everything you do will put you in jail for years. And yes its restricted but not banned i can think of several reasons why you would be allowed to own one (bounty hunting, mercenaries, etc). Mind for each one of those i can think of dozens of reasons it probably never see light of day.
The question was "what is the best rifle?" I'm still gonna say Heavy Repeating Blaster
The best ranged heavy unrestricted rifle? Heavy Blaster
The lack of power creep on ranged heavy still makes it one most powerful man portable weapons. Fully modded with Augmented Spin Barrel it out performs the Sidewinder, potentially having 13 base damage accurate 1 and pierce 1 to go with its autofire while still have two hard points left.
Honourable mention Bowcaster its mod options arn't quite as good as the Heavy Blaster and it lacks the extra HPs
For stun damage ACP Repeater (if bought stun only purchasable at char gen) for autofire or raw damage the LBR-9 (expensive) or HBt-4 Hunting Blaster (if bought stun only purchasable at char gen)
You not going to use any of them in polite society but honestly i shouldn't have to say that