As far as the 75 minute length vs. classic series multi-Doctor episodes, those classic eps could do quite a lot of dragging on.
If they pack a lot of action into 75 minutes, it'll be good enough.
On the other hand, many newer DW stories seem to rush by too fast with too much jammed in! Seems like you jump from the setup to the conclusion without much plot inbetween!
A little more time would give these stories room to breathe! 90 minutes would've been ideal.