I have heard it from multiple people over the last 5 years that Optoisolators are failure prone and should be avoided when able. I'm working on a project (for personal use) that I was going to put an optoisolator on to isolate a logic system from a 170V rail. I do plan to use this device for teaching and would hate for it to stop working because an isolator failed. I know there are magnetic coupling ones now that may be better?
The real question is that I have not been able to find any real data on the internet that confirms optoisolators fail easily or with respect to other solutions.
The only thing I found was a report by Sharp Found Here.
I'll also add all I need to do is move a square wave pulse of 100-500kHz from a logic supply that is floating to a -170V rail. So it can tolerate slow edges and such that a digital com system could not.