The Reaper's a more powerful version thanks to Twin-Linking.
Weapon power isn't ENTIRELY connected to rarity. While particularly powerful weapons are generally less common than a lasgun, an Autocannon is a relatively simple piece of hardware, while heavy bolters have the whole "but only special people are supposed to use this" stigma that comes with it. Heavy Bolters DO provide significant damage output, particularly with their special ammo options, though.
Its better shown in Rogue Trader, where acquisitions include the ammo for it on a permanent restocking basis: Bolt shells are expensive to manufacture, and having a bolt weapon is a symbol of having the influence, finances or logistical support train that one requires to actually keep that thing well fed. A Trader with a Boltpistol or Exotic Weapon is making a statement where he stands.
The autocannons are no worse than a multilaser, lascannon, plasma cannon, meltagun, force weapon, anything swung by a marine, or any other number of things that can be tossed around. Boss fights can be rather brutal for that reason; when both sides are tossing around multilasers vs bolts of change, someone's gonna be burning infamy and fast. But no, its not a "terminator armour only" thing, neither are most other weapons, either.
Its heavy, you need bulging biceps or special equipment or cyberware to fire it without bracing, but its perfectly usable by an otherwise unaugmented human who can actually lift that kind of weight [he'll probably be quite encumbered if he's got nothing to compensate however].
Don't forget about shields, dodging, cover and other useful things: you need to be supremely confident to NOT at least try to put a wall between you and the not-exactly-hidden heavy weapon you're about to have to deal with.
Oh: personal favorite: Integral components turn that multilaser into an exotic weapon [integral weaponry], which makes it compatible finally with weapon-tech. The 'simple enough' multilaser can become a spectacular legacy weapon earlier than many other things. With the right pattern, it can even start wrecking vehicles, or utterly devastate a horde.