I've worked in a financial company running on mainframes. They were the least reliable part of the whole organization. The only reason they're still around is they've become hostages of the mainframe. My experience is that nobody actually likes the mainframe or respects it. They just tolerate it.
Being held hostage I can understand and see that point of view.
When it comes to IBM z/series not being reliable though
that is certainly not my experience.
Do you know which generation this was?
Were they quite old?
I'm sure the mainframe itself is ironclad. It's the programs that crashed all the time, the fact they couldn't scale the load to match growth in time, they couldnt get developers to make updates at the volume required. As a platform for the business it was as stable and reliable as sand in a hurricane.