One more thing, regarding Small Forwards (one of the speeches mentioned this as a lack. EDIT: it was actually clubber_lang's pot above).
Having a SF for the U21 is nearly impossible, and widely acknowledged to be so (it is false that managers are not encouraged to do this).
I just want to chime in on this as well, but first on the comments above that SF's are "undervalued" and "not encouraged". This is kind of crazy--for anyone that follows the TL at all, it's obvious that good SFs are the most overvalued asset in the game. This is true of the NT and u21 teams as well... we almost always encourage people to take well rounded training routes to increase the likelihood a player is able to be used at SF in some capacity down the road. For the NT, training a guard to get on the team at this point is frankly very difficult (with guys like Bronson and Madrid around), so making a SF is much more practical, since there are more types of players that might be appealing. In fact, we have a whole section of the offsite devoted to team training logs, and just about everyone that posts there is working on SFs.
As to the difficultly, I think it helps to put some numbers on the issue. By the start of the age 21 season, a really well trained player is likely to have gotten 30-35 pops. A well rounded SF needs at a minimum, JS, OD, IS, ID, Rb, and ideally Dr, Hnd, Pa. Even if you had an amazing 18yo prospect, with 7's in all skills, and absolutely perfect training (and perfect luck, no injuries, etc), your player is going to end up deficient (meaning a skill <13, and even 13-14 can be a liability at times) somewhere. So the question is, do you want a guy with only 11-12 in JS (which eliminates outside offenses), OD (which eliminates the 3-2 and leaves you vulnerable to outside attacks), ID/Rb (which leaves you vulnerable to those teams that still use 3 bigs), or horrific peripherals (likely meaning lots of turnovers and poor passing)? Or is it better to get a couple specialized guys (say, guards with slightly better than usual rebounding, or bigs with decent jumpers and OD) and just pick whoever fits your game plan best on a week to week basis?