I prefer PNF as well.
I've often challenged crew in the sim as to why they are not actively monitoring the other pilot - that is, an obvious speed deviation or other such thing. Quite a number of times I've had the student(s) turn around and tell me they would do it on the line, but they didn't want their sim partner to get "caught out." Especially so on the rides, where I've only twice heard some one call deviations. Often it's more of an "under the breath" comment that they think only they can hear.
I'm also training a wide range of experience - from 250 hours all the way up past 20,000 (where I'm not quite too sure who is training who some days). There is often just as much reluctance on the part of the experienced pilots when doing sim with someone with much less experience for fear they destroy confidence.
From this standpoint, I think we do need to re-evaluate (as trainers) how we are teaching what the PNF's job actually is and how best to communicate that a speed deviation call is much more important to catch than, say, making sure all the paperwork is complete. Unfortunately, way too much emphasis (of which I'm guilty myself) is put on the "learn all the SOP's for your ride" during an initial, which naturally carries through into the rest of their career.