Flash was a massive security hole. It was a resource hog. It was based on proprietary Adobe technology. It was woefully unstable and during some periods caused crashes frequently even on powerful desktop systems.
To say that maintaining iron-clad control of the platform was the only real reason Jobs chose to block Flash is almost insultingly reductive.
Ironically, most of that is now true of web browsers (or should I just say Chrome at this point?)
Chrome is a privacy risk. It's not a security hole in the sense that bad actors can exploit its horribly buggy code and gain privileged access to your system (at least, not commonly that I've heard of).
Chrome is proprietary, to be sure, but it's heavily based on the open-source Chromium.
I'm not aware of Chrome being massively unstable on any common system—however, I freely admit that as I don't use it myself, I might just not be noticing the complaints of others because they don't show up where I would see them.
...Chrome is absolutely a resource hog. No argument there.
They would have had years of crash telemetry from OSX and so would have been intimately aware of its stability.