First, the further out you put your receivers, the longer the throws you are demanding of your QB. That requires velocity and accuracy. Hooker was good at both, and particularly accurate deep.
But in games where his deep ball was off (particularly the South Carolina game) suddenly they couldn't hardly complete any passes at all. Milton has the velocity for those longer throws (which keeps DBs from undercutting them), but he really struggles with the accuracy.
Ironically for this week, Heupel runs a great scheme for someone like Milroe. But a guy like Stetson Bennett would really struggle in that offense (as he doesn't have the tools for those long and deep outside throws).
An issue is that you are asking a ton of your OL. The wider the splits, the longer throws take, and the less help you have for edge blitzes. This is why the run game is so important - you have to have a constant threat of the inside run to hold the LBs between the tackles.
If you can't run, the defense has the ability to attack off the edge and can easily overload one side or the other. And if the defense is allowed to play 2-high safety, you have to be more cautious on deep shots or risk INTs, and that cautiousness leads to pressures.
Tennessee has two problems this year. First, Milton is rather inaccurate. Comp% has only dropped about 10% from Hooker, but that's due to higher reliance on screens. Downfield throws have become far less consistent, which is an issue as 2 misses in a row = punt.
Second, Tennessee's OL just isn't as good. Hooker benefited from some elite OT play that gave him far more time than expected from a 5-man protection on a passing team. Without that protection, deep routes aren't as likely to succeed.
Heupel's offense is a great scheme IF you aren't at a talent disadvantage. The one team that put them at a clear disadvantage last year (UGA), destroyed them. But UT's 2022 squad had elite QB, OT, and WR play. 2023 much less so.
UT is a "run team" this year because of how they're played. The goal is 1) keep 2-high safeties outside your own 40 to prevent cheap scores, 2) know that Milton will throw away a possession or two with bad throws, and 3) clamp down on the run when facing a compressed field.
Ironically, there's an argument that Bama could run the offense very well with this year's personnel. But they likely wouldn't want to because it asks so much from the QB, and on a day where your QB (is cold, they lose a winnable game (see SC loss). Bama doesn't want 10-2.