Yes, as you've all said there's a body frame limit put in place. Most radical I've gotten is reducing a TE over time to WR. So, figure a 6'3" TE trying to descend to 198 lbs. I believe sometimes it halts on the way down to somewhere between WR and RB weight. Also, it'll take awhile to get there.....down to RB, for instance.
I've never tried to reduce a 6'4" or 6'3" d-lineman down to CB figuring the body frame element would impede it or make the CB ineffective due to over-size height.
Flip side, I know JDB and the Gang got mad and made it so that DBs *** rushing the show at DE got mashed and crushed so I try to be careful with testing limits.