It's not the Nature Of VoIP.
It's the Nature Of VoIP across the Internet (rather than a private network).

My employer uses VoIP on a very large scale (Hundreds of offices including Asia and the UK/EU). Its QoS is like POTS, because traffic does NOT flow on the wild-west of the Internet routing.

VoIPo, In My Experience, is near average in quality/reliability, a bit overpriced, slowly improving. In the running. Not yet way above average.

But really, the combination of the Internet and use of Asterisk (which I assume) mean that no cottage-industry VoIP provider can excel. But with the slow demise of the US class 5 switch POTS, this is the future.

I don't rely on VoIP - just testing the waters.