The problem of reliability isn't really with Home Assistant. It comes from the pain of integrating the almost-but-not-quite compatible cheap Chinese devices that claim to work with a standard, but don't.
The ecosystem is so fragmented between cheap race-to-the-bottom gadgets and the more expensive "works only with OUR app" that integrating anything more than just a few bulbs is a pain.
And that's not even considering the amount of customer support that someone doing this professionally would have to provide.
Why would installers deal with cheap Chinese devices? Every installer I’ve dealt with has a relatively small selection of vendors and devices that they know and trust. They aren’t reselling random crap from Amazon.
No, they're each having a relatively small selection of vendors re-selling white-label Chinese crap.
In my case: local A/C installers are reselling Haier aircons. Good hardware. Shit software (see "tale of the two apps" elsewhere in this thread). Or, a nice local company selling floor heating solutions. Got electric floor heating from them. Control panels have "smart home integration". Guess who made that? Tuya. The world's finest seller of white-label "smart" devices. Complete with a shit app.
Home Assistant is the only thing that makes the two device classes more convenient remotely than through an IR remote or using the wired-in control panels.
Right there with you that the amount of customer support needed for such a service kills the incentive. I already hate having to fix my house when something breaks or a battery dies. The last thing I want is Joe Schmoe calling me up at 2am because his power went out and shorted the home server, and he has no backups because he disabled the ones I set them up with while "tinkering"
I've had plenty of trouble with Home Assistant upgrades randomly breaking smart devices from well known vendors, e.g. whenever I update I dread to find out whether my Neato vacuum robot is still supported or whether that integration has crapped out again. Same for my LG smart tv and several other devices. I love Home Assistant but it's not exactly rock solid...
The ecosystem is so fragmented between cheap race-to-the-bottom gadgets and the more expensive "works only with OUR app" that integrating anything more than just a few bulbs is a pain.
And that's not even considering the amount of customer support that someone doing this professionally would have to provide.