Hm, so I was a little bit hasty. I was able to launch with that option but the first combat became unplayable with pink boxes everywhere. But the interesting thing is that I've tried to change graphical settings. Afterwards I could run the game without that option. Then I erased the whole game's directory in /home/user/.config/unity3d/Timeslip Softworks/ with the saved settings. And now 64bit executable launches with or without -force-opengl no problems and no pink boxes. I tried a few missions yesterday and even on highest graphical setup it was in good fps space and running well. -force-opengl uses the legacy OpenGL back-end. Some of the folks having this issue also had older machines, but that's not entirely this case. I have GeForce GTX 750 with latest stable nvidia drivers.
Because it was late and I wasn't properly paying attention I cannot say for sure, but it's possible that I had some old setting files in .config folder from the times when I was trying demo during Kickstarter, even when there were no saves anymore. Then when running with -force-opengl I was able to rewrite said settings. But in the end, erasing that whole config directory might be the real solution here. Unfortunately, I didn't checked it before and cannot reproduce the problem nor make any diffs. Sorry for that.
Unity together with mesa and nvidia drivers situation in linux environment isn't easy nor feasible for me to delve into. How much you can do on your side, I don't know. As in this case there is simple possibility I was just stupid and having old files.
If (big IF) the old configuration was the culpit here, you can make the checks when loading from that directory more conservative and if something doesn't fit quite well rather decide to set everything to default? I mean better make players change the settings again than having possibility that it crashes on sigsegv.
I understand this is pretty much wip. if the development is wild, I dunno, easy and sure to do is perhaps temporarily making a new directory for each game's version? Like 'Vigilantes14'. Especially if you're planning updating to newer unity versions and/or having incompatible saves anyway.