FCM or Open UART Interface?

Very true!
But I would certainly recommend that anyone try FCM first - and only resort to Open-UART if FCM proves unworkable.

This is true.
It does impose a performance overhead - so that could be another reason to choose Open-UART

True - but you also have to do this for Open-UART.
See: AT+WMFM - Inconsistent (and unhelpful) Behaviour

Very true! :open_mouth:
Not only in terms of the target operation - but also in terms of the awful documentation! :angry:

Surely, if you want to implement a custom AT parser, you could do that using FCM in Data mode?

Switching FCM to Data mode bypasses the AT Command processor - can you explain what you mean by this?