I think WR Cage's comments are very helpful and important.

I was once on a YHZ-YUL-FRA-FCO trip and misconnected in YUL because of a thunderstorm. The transfer desk person was very nice but clearly did not know what they were doing. As soon as I got to ticketing, they revamped my whole trip (YHZ-YUL-YYZ-FRA-FC0).

I think the key issue is that some working at the transfer desk know their limitations and refer on quickly. With others, you can spend hours or be lead down misleading paths, because they simply don't know.

In this type of circumstances, I have never been disappointed by going straight to the ticketing desk, even if it means going through security/immigration again.