The beam is just a name of the shell type though -.- It isn't an actual beam. Just one shot that succeeds in 2\3 of attempts. Which is only barely comparable to ravager. Not to mention the fact this one is absolutely useless when jinked, unlike ravager with its 3 hits.
Good points, though I think I'd still lean towards the Reaper over the Ravager. Even if the ravager should hit with 2 shots on average, one of those shots has pretty good chances to fail to pen/glance. Against Tau, Eldar, and Chimeras against front armor, it's about half the time. So an average of one shot does damage against those. And then the same vehicles that could jink against the reaper can jink against the ravager leaving you with 50% (assuming no holofields, nightfighting, etc.) of that final shot *still* not doing much damage.
The reaper may be less likely to succeed at at least one to-hit roll, but (without doing any real math-hammer) I get the impression that it's a bigger payoff when it does
hit. The strength 7 shot isn't terrible, but it's almost an afterthought. You're going to be doing about 2 haywire hits so long as you hit the enemy at all. That means that an average succesfull hit will either wreck a 3 HP target you've already taken a hull point off of or leave said target ripe for the finishing by another unit.
Also, I tend to not be especially worried about jinking. My local scene has mostly imperial vehicles, and most of those don't jink. The necron player is the only one whose jinking is really problematic but (even with tesla barge shenanigans), I'm usually happy to miss with a shot if it means he's only hitting me on 6s on the following turn.