it works. 4.0 gives you transactions.
it's pretty great.
and, most importantly, as far as I know the default replica set config is safe. (durable, consistent, atomic) it handles a lot of failover scenarios for you automatically. and you have to manually intervene to get into a bad state - which might be okay for that particular business case (better than full downtime)
Yes. It is so easy to bring up a mongo cluster and feel like you have HA. Don't worry it won't be proven wrong until you have writes during your cluster degradation and are unlucky.