Back in the day, both the Mac II and Mac IIx machines were exceptionally reliable - in my experience, anyway. The only common problems were the PRAM batteries going flat (and then the machine wouldn't power on) and floppy drives filling up with dust. Apple later made an add-on clear plastic "dust boot" for the floppy drives to fix that issue.

My employer at the time had a Mac IIx running as a dedicated AppleShare server (with Ethernet and all - a very high end machine in 1989!) that was powered on continuously (except for the occasional power blackout or scheduled downtime for hardware/software upgrades ) for around 8 years without a hardware failure.