In my experience, wi-fi calling works as if on the home network, regardless of IP location/endpoint. So this would work similarly with an eSIM.
Fortunately, as far as I remember at least iPhones route VoWiFi traffic over a VPN, if any is connected, so that's one way to still use it abroad.
The Vo WiFi spec only defines handover procedures for networks that implement VoLTE, but VoLTE roaming was historically almost nonexistent, although the situation is improving somewhat.
This means that if you go out of range of your WiFi router, your cell phone has no idea how to request a handover from the network, and the call drops.
Also, I'd much prefer a call getting dropped to suddenly changing from being included to billed when traveling abroad.
For EU carriers roaming used to be a big business (Europe is probably the region where most traveling abroad happens, many small countries close together without travel restrictions). The EU carriers don't seem to be able to let go of crazy roaming charges (5€ per minute and more are not uncommon outside of EU).