The lack of updates is alarming (I like to think they're waiting until they iron out the progress loss issue before updating here), but calling it unplayable is a bit much. The trick is to make a backup of your save/progress files before entering a level, and then restore them if you can't finish the level properly for whatever reason.
The progress loss bug is apparently related to the save decryption system, so long as you never let the game try to reload old data (like after a character gets captured or you exit a level early) there doesn't seem to be any risk of progress loss. Most of the level hang-ups can be avoided as well by simply not fighting in/through doorways.
The game is absolutely buggy as sin and takes some effort to safely play, but it is not unplayable.
I appreciate the tips, but, honestly, there's way, way, more broken here than just their save game system. If you get far enough into the game, the GOG version features broken "gold" skills (that reset progress on game launch), and the level 30 dungeons come up hopeless b0rked about a third of the time. And that doesn't even mention half the monsters that possess "lock up the game" as a playable skill. (I know, I know... just kill them quicker / find a way to stop them from using it) All of which is theoretically fixed in the current steam release... which we don't have.
Not to get all philosophical, but at some point for everyone, a broken game becomes unplayable. I mean, if I really, really wanted to play a working Rezrog, and I had infinite time and patience, I'd just bust out Unity and rebuild the thing from the ground up. Clearly it isn't worth that much effort, and at some point, even investing the time backing up save files and walking on eggshells in game to dance around game ending problems... only to have to restart the same dungeon 4 times in a row because the game is a mess just stops being worth the effort. I'm glad your getting some enjoyment out of it, but, for me, this needs the recent updates from steam to even try to play.