Good meta question, but I don't think there's a way to ask what you want to ask on the main site without it getting closed.
The reason is that there is no universally accepted definition of "reliable". In fact, depending on exactly what you do with the software, and the platform/version you're running, it may be the case that two users with different use cases have completely different reliability: one person might view it as perfectly reliable with no bugs whatsoever because they simply aren't triggering any code paths that contain bugs, whereas another user might hit every code path that has a bug.
Not only that, but each user's threshold for what is considered to be an acceptable amount of bugs in software is highly variable. I'm sure a LibreOffice developer has a very different idea of how many bugs they can tolerate while still getting work done, vs. a first-time computer user. The former will probably be able to work around even major missing features, whereas the latter would get "stuck" upon a simple problem like the absence of a tooltip.
Finally, since OpenOffice is free to download and use, really the only way to "tell" if it's reliable for you (for what you use it for, on the platform you're running, with the hardware you use, etc.) is to just download and try it. There's really no better way to judge its reliability than to spend some time using the software and see it for yourself. If it were proprietary and expensive to purchase, it would be understandable if you wanted to ask others' opinion of the software before buying it, but even in that case, it would still be off-topic for Super User.