When a TCB distant signal is placed before an ETCS (or any other cab signalling) sign, the distant signal will handle the ETCS stop sign just like a normal tcb signal, which includes distant reservation expansion, which is is especially troublesome when using many-aspect signals.
The other way round is even worse: In cab signalling mode, any distant signals are ignored, thus the first signal after a cab signal will always require a signal to be placed in a locatoin with very good sighting and will still slow trains down due to maximum sight distance.