For the other chance to happen effects, there was no limit on their usage (e.g., they were passives for the main gun, etc). For this effect, you have to use missiles, so it will happen less often.
As for its brokenness, it depends on the frequency of antigravitate blockages. As a reference you were saying you get 6 credits (on average) (~12 if you are patient) for a blockage? If that's the case, then yeah, sometimes you are going to get a bunch of credits (255+ qualifies as a bunch, also you can spend infinite credits in the shops buying experience, so you don't have to leave levels with credits wasting away). However, how often is that going to occur? I don't have the numbers on me right now, but the frequency of antigravitate blockages is pretty low, perhaps 1 in 10? So on a typical run, I'm not sure you'd see something that large. However, the rooms that have anti-gravitate blockages also tend to have several of them, so you'll probably see most runs with a very low count, but on a few you'd have a ton of them. Right now, it'd guess maybe there is 1% per floor of getting 255 credits? I'm not sure if that is broken, simply because it is very rare.
One of the things that Pepisolo is always bugging me about is how he loves chaos. Or as I like to call it, variance in outcomes. He wants to have infinite various all of the time, whereas I want a more structured approach. If this does only happy 1 in 100 times, then I don't think it is a particularly big deal. On the other hand, it it happens 1 in 4 times, then that's probably too much. I just don't have frequency information on it.