I think you can learn a lot from bullpen sessions if done properly.
You can't learn some of the things that Hillhouse pointed out and that has an awful lot to do with outcomes in games.
I see people calling pitches in and out for kids that struggle to throw strikes and they may throw gas. We have a kid that throws pretty hard but is consistently wild in the zone and pretty good. If you try to make her throw in and out early in the count she is always behind in the count. Of course you are going to work on those things in a bullpen but understand what she needs in the game.
Now we have two that can live one or two balls off the plate or on the black with FB, Curve, change, and drop and are not hard throwers. You have to call pitches off the plate for them or they just throw it over the plate. For example, 0-2 and you call curve out, most of the time it is on the black. You call curve one ball off and you get a swing and miss in theory. Bullpen sessions should be about what the kid can do and how to enhance their skill set.
Also, they may throw a great change 15X in a row but can't mix it with other pitches so make them mix everything in a pen. I have one kid that throws hard and has a nasty curve, when you mix them her FB after curve cuts and she normally misses out.