I did a fair bit of googling on this as it was my first time using eSIM. Apparently for iPhone the network switching eats the battery, whereas for Android (which has had dual-sim support for ages) it’s not a problem.

Nothing else changed in my usage, and eSIM only worked fine, it was just when the physical sim and the eSIM were both active that the battery drained super fast (like more than double the usual rate).

YMMhaveV of course.