I'd be curious to hear about this too - I don't see any issues filed or commented on by him, perhaps the bugs are more perceptual mismatches with his React experience as opposed to actual bugs. We only recently released preact/debug, which brings with it some of the guardrails React ships by default - maybe without those people run into issues.
FWIW there are hundreds of companies (some very very large) using Preact in production. If there were showstopper bugs, they'd be fixed immediately.
I think this may be referring to "preact-compat" issues rather than Preact issues. I know the Inferno team recently had to change a lot of the state scheduling to be 1:1 with React 15/16 (i.e. batch sync schedule setState after events was very broken and there were issues with controlled components in Safari).
FWIW there are hundreds of companies (some very very large) using Preact in production. If there were showstopper bugs, they'd be fixed immediately.