Yep, 100%, but that's why identifying compatible workloads first is key. A lot of orgs skip right to the savings pitch, ignorant of how their applications communicate with one another - and you hit the nail on the head that applications doing even some processing in a cloud provider will murder you on egress fees by trying to hybrid your app across them.
Folks wanting one or the other miss savings had by effectively leveraging both.
Any experience with the mid-to-small cloud providers that provide un-metered network ports and/or free interconnect with partner providers?
(For various reasons, I just care about VPS/bare metal, and S3-compatiblity.)
I'm looking at those because I'm having difficulty forecasting bandwidth usage, and the pessimistic scenarios seem to have me inside the acceptable use policies of the small providers while still predicting AWS would cost 5-10x more for the same workload.
Vultr and Digital Ocean both offer Direct Connects. I've had good experience with their VPSes.
Netcup and OVH provide free un-metered ports. There are actually lots of options available on the market. BuyVM is another good one.