shoe - when it comes to matters of substance about the actual game (how to recruit, coach, etc) - you are doubtlessly one of the best posters on these boards. your information tends to be very good and you bring a non typical perspective which i think is fantastic.
this one though really misses the mark. you say fb/fcp is unquestionably better with uptempo, but that is tremendously wrong. the pace of fb/fcp normal tempo is so insanely high already, that it is virtually impossible to exceed the range where increased depth adds major value. 85 stamina with a perfect 12 man rotation is not at that point, that's the best i've had personally, and foul trouble and fatigue both remain serious issues at normal tempo.
meanwhile, the opponents range the spectrum. some will have substantial problems against a fb/fcp normal tempo, others have the depth and can handle the high pace fine. there's a messy middle there, but in short, the way things work out is pretty simple. the teams who already struggled facing fb/fcp normal tempo, they struggle more against uptempo, and both sides (yours and theirs) take a meaningful hit on matters of depth. maybe that tradeoff is worth it, maybe its not, and probably its situational.
on the other side, there's the teams who can handle facing fb/fcp normal tempo pretty well. there's a bunch of these, and what i always found is, if you are facing some 11 deep man team who can take your fb/fcp tempo like a champ, going uptempo is not on average causing them much trouble. a big part of the reason fb/fcp (and press by itself) are so powerful is because they possess the means to take advantage of all of the depth they have, while man and zone get vastly less value in that season from their 11th and 12th men. similarly but conversely, increasing the tempo on those reasonably deep man and zone teams is not going to hit them that hard, and for the really deep ones, it barely touched them. yet it always is a meaningful hit for the fb/fcp. in this case, the tradeoff is clear - its worse for the fb/fcp team - and the better the fb/fcp team, the more foul and fatigue trouble matters, and the more clearly worse it is (when you say its the really good fb/fcp teams you uptempo with, and the ones with deficiencies you run normal with, you have it 100% backwards).
fb/fcp is the ultimate high pace scheme, but that counter intuitively makes it the absolute least viable scheme to run uptempo with against quality NT teams (top 20 maybe, for d1 levels of competition), because the fb/fcp team is virtually always taking the bigger foul/fatigue hit against teams of depth. if one wants to use uptempo situationally, i think that's reasonable enough, but the expectation should be that most quality NT teams are going to have too much depth for uptempo to work out favorably. literally, there is no other combination in this game i would be less likely to run uptempo with in a NT game against a quality opponent.
even straight press has all the same issues, and for all the same reasons, i rarely run uptempo there - especially at the highest level! all the huge title favorite teams i've had have basically all been straight press, its always been my main line, and its like 100% normal tempo all NT. its more viable when you aren't a favorite, so concerned about foul and fatigue trouble. also, the man and zone teams with solid depth need to think about running higher tempo against press and especially fb/fcp teams, because the increased foul and fatigue trouble almost always favors you (there are other considerations - i'm not saying just to start running uptempo into these teams all the time - but at least to think about it!).
quick note for anyone who does try running the higher tempo into these press and fb/fcp teams - when things go bad for you, they tend to go REALLY bad. these are the times when foul/fatigue wise the press team gets lucky and has no trouble and then they are like insanely good, and then you might have gotten bit by it yourself, and you just get like 25 TOs and lose by 30 and its like OMG. but don't be too scared by those cases. you were going to lose all those games anyway at a lower tempo, when the depth-utilizing teams (press, fb/fcp) don't get hit by foul/fatigue they tend to be pretty OP.