I don't mind a stable DOS system.
Scene: a US high school, opened on 6 Jan 1996, freshly built with all new networks and shiny computers in the main lab running Windows 95, plus a handful of Macs. Yet one computer in the main office was still running only DOS [1], and had to have 24/7 uptime... because it ran the fancy 16 X 80 incandescent Daktronics sign out front, the first for any school in the district.
Said sign turned lots of 'leccy into lots of heat, plus sufficient light to be distracting to the commuters headed for the nearest major metro (almost blinding at night). It wasn't the best Big D had to offer, but on the district's budget it was the best they could do. And the master software [2] ran in DOS -- I personally ran it my senior year (12th grade). I could manually run sequences or load a full month's worth (or more) into the schedule, including on/bright/dim/off commands. So while the sign was off, the DOS box still had to run all night to wake it up again in the morning and command the appropriate sequence(s) for the next day.
It mostly ran on a (small) hard drive, but it did have a 3-1/2" drive (not 5-1/4), and loaded other software just to see what the machine could handle... without messing up the current operations. [3]
Good times...
[1. Admittedly not actually that out of date, yet. If it's still there now, that would be a different story, but I assume they've upgrading the sign and the computer since then, probably full-color, full-video LED.]
[2. Actual sign ops were in a separate box screwed to the wall, connected by serial. This ran only the current sequence from its own memory and needed the PC to give it the next instruction/sequence.]
[3. Because of #2, the PC *was* allowed to reboot -- and the control software called up from autoexec.bat -- but you had to manually reload the Scheduler. This was definitely NOT preferred and usually only happened if power was lost, in which case the sign rebooted blank, which was (marginally) better than some default pattern or re-running something out-of-date.]