Its free and open source as far as i know (you mentioned affordable). The drawbacks i've had so far are on bad routers (double NATs) where it gets a bit sticky with connecting, otherwise all quite good, one of the best pieces of software i've ever used.
If the configuration when connecting behind a NAT on a badly configured router i've never really had a bad experience with ZT. I use it to connect into my AWS VPC then connect to anything I need to as if it were local.