1) Slow RTT
2) Misuse of bitcoin with crazy amounts of blockchain spam (could end up blocked by miners too)
3) Expensive
4) Loss of access to a Bitcoin API means app downtime
5) etc...
From the docs it does not look like that every http tx is stored onchain. It is more like an ssl-alternative where you sign stuff with bitcoin crypto and therefore not need to trust the sender.
Maybe it's adequately named 'vapor' as in vaporware? But I agree, surprised it didn't implement its own token or cryptocurrency instead of polluting BitCoin.