When I found out that Legos are made of ABS, I knew it would be fish safe. I've seen lots of ReefKeepers with their sensitive corals use it, so if it hasn't caused any problems there, it won't be an issue for our fish. I don't think pressure is an issue for anything we would be using it for. The manufacturers suggest it be used for non-pressure applications only for the same reason they say it's not to be used for potable water...satisfying the requirements (testing and certification) to make those claims is expensive, so they don't make pressure or "safe for potable use" claims. It's plenty strong, but you're right, not as strong as PVC.
Another cool thing about it is that sweep fittings are much more common, so the plumbing is much more efficient (less head loss due to friction).
