I guess Clipper wasn't super reliable when it came out(?) (so long ago at this point it's hard to remember), but I don't really remember having much fondness for the old BART cards. (Not to mention the annoyance of having to pay for BART and Muni in different ways, or carrying around those silly paper Muni transfer slips.)

These days, I can't remember the last time I had an issue with my physical Clipper card. I do recall lots of issues when they first rolled out the ability to put a Clipper card on your phone, but I haven't run into a problem with that in at least a couple years.