Other than a weird glitch in which a stereo upgraded to 'unbreakable' did break and couldn't be fixed (I had to 'replace'), my non-breakable items have worked as advertised. Some objects can only be upgraded to not-so breakable.
I've noticed that when you have a Sim working on the handy trait, things seem to break more often, as if to give them more opportunities to work on their new skill. I had a Sim with full handiness upgrade everything in the house a generation ago. Her granddaughter is now working on her handiness, and the shower breaks all the time. It's just about the only thing that didn't have the 'unbreakable' upgrade option.
Mostly, though, I don't mind things breaking - it adds a little variety to my Sims' lives. (Although, in un-upgraded households, the toilet never fails to break just before bed. I don't know how many times I've had an exhausted parent fixing the toilet at 2 am.)