First off; make sure you're looking at the Errata weapon stats.
Secondly; Lascannons are damn good weapons in fluff. Most Guard vehicles mount one, most Imperial Navy aircraft have a few of them twin-linked, and they're the go-to weapon for taking out vehicles. You're looking at something made to blow up tanks and shooting it at critters.
That said, it doesn't have a Full-Auto option, it can't use specialized ammo, it has a small clip, it takes up both hands, and the fella firing it is making himself the biggest threat on the battlefield. The first time your Devastator fires it, expect every bit of incoming fire to focus on taking him out. It also isn't much good against Hordes, as you said.
Good, but not OP. Every weapon has its place.
My apologies to anyone I offend; FFG staff, playtesters, and forum users alike.
Please check out my Dark Heresy to Only War conversion! You can find it on the main Only War forum. I'm always looking for more people to playtest it!