MC is almost always "beta" as it's constantly being developed. Any given day, something could go wrong, but it's very unlikely. The big changes are tested by the beta team before being released here, so any BIG issues are likely to be caught, and kept out of the hands of the public.
I've been using v17 for the last several months, and have not had any major issues, and it pretty much never crashes; so I think it's fair to call it "stable" from that point of view. However, since it's in constant development, things can/will change, and the changes might not always be ones that work well for everyone, so it's implied in the "beta" label that you could have some 'growing pains', but generally they aren't going to bork your setup.
With that said, be sure to backup regularly, but know that MC is doing this for you also, report any niggles you encounter, and enjoy the ride
