In exe.dev you have a subscription and you get some constant compute, paid whether used or not. that compute is shared between the vms. In shellbox every box has its own dedicated compute and if you don't use any, you don't pay.

Shellbox also gives an ipv6, an email endpoint, wakeup on the web endpoint, it supports nested virtualization, docker, etc. And... it is much cheaper even if you use it 24/7. Ahh and also you can send ssh exec commands, ssh forwarding, and more. It is "real" linux, like you get on classical vps providers