I believe Mosquitto is one of the most robust apps out there. Running it on other 'suitable' hardware is going to be near 100% solid, setup and forget.
I really don't think it belongs in the HE Hub although I agree it's tidier. I'm very much against that option, and the folks in HA land have also backtracked from that inclusion.
A broker 'appliance' is a useful and important device in your home automation setup and users who need an MQTT broker almost by definition are using multiple devices (clients). Making that dependent on HE stability seems to not sit right, particularly if you have issues as some still do.
Raspberry Pi with SD is always vulnerable but there are other options, and Pi4 can now boot from SSD which is much more reliable. I have had SD cards fail on Pi but it's expected and you backup around that scenario.