I have seen the opposite the case sometimes, too -- people overcompensate and treat their friends/spouses worse than they'd treat an arms length employee.

I personally worked with a married couple as founders, and would never do this again (a lot had to do with them in specific, but the general failure mode of any relationship problems spilling over into business, on top of them always ganging up on others, seems general). I'd probably work at a company with siblings as founders, and maybe at a company where a couple were in different roles (founder + employee).