I should probably say that I have a dislike of (and am biased against) MikroTik because of their disregard of the GPL. For the price, though, I think the hardware is generally pretty decent.
Anyway, I don't feel real confident in the security of the RouterOS software although I don't have any hard or articulable reasons for that. It's just a "gut feeling", I suppose.
I do have much more confidence in both OpenWRT and FreeBSD. If you're just using the device as a home router/firewall, you don't really need many (if any) daemons running and exposed so the attack surface is pretty minimalized. My own router at home came with RouterOS on it (although it's not MikroTik hardware) but I replaced it with OpenBSD.