I've played a lot of missions using build 18.3 and got up to the late parts of the game, with very big ships and powerful weapons.
However I think that while the first part of the game is quite stable, and only a lot of balancing is needed, the same cannot be said for the second half.
When carriers and mag weaponry start to appear, I've had a lot of ground combat crashes and freezes during the alien ground combat turns. Moreover, a lot of crashed ships with no aliens, very frequent crashes while loading ground combat saves, and so on.
A few bugs are also present in the geoscape (eg an alien terror mission on a city in a continent that had already signed an alien pact), but geoscape is quite stable now.
When all the issues with the archiving system will be solved, couldn't we have a developer provided savegame in the second half of the game, a few months later with a couple of bases already built? In this way testing the second half of the game could be way easier.