The good:
Maddening generally forces you to mind your positioning, and the enemy scaling puts up stiff resistance, so I think that in general terms, it works well. I do like the choice to give consistent bonus abilities to all enemies of a given class, it eliminates those situations where forgetting an enemy has a skill results in a lethal mistake, which caught me out a few times in Lunatic+ in Awakening. Making linked support bonuses and gambit boosts more relevant is a positive change that adds depth to the game. The adjutant guard feature gives you a mid-late game option (on NG) which is more relevant to mitigate the high enemy attack speeds, but can't be used on a flier, which makes that choice more interesting. Due to limits on the casts of magic, it is no longer quite so easy to exploit a healing class to overlevel and outscale / trivialize content on the hardest difficulty just by spamming heal, which is a welcome change from prior titles.
The bad or indifferent:
I'm not a fan of ambush spawns, but divine pulse takes the edge off of that, and I have to admit giving the player a turn to uncounterably kill them wouldn't be as challenging. I think BE Church chapter 18 handles this well by telling you a turn before they are coming. I wish the game did that more often. I kind of wish they gave every boss the counterattack at any range skill, or at least a 2 range option so that they weren't so trivial to kill by outranging. There are still numerous degenerately powerful tools (avoid stacking, encloser spam, impregnable wall, etc.) that can trivialize parts of the game, even in Maddening, so I wouldn't call that balanced, although it is tactical. I think some of those powerful tools were considered when scaling the enemies, which makes Maddening a departure from the balance of simple unit vs. unit combat (like FE 6, 7, 8 ), and since it is often hard to safely win in a straight fight, it turns into more of a game of uncounterable damage or something like a strategic exchange of tactical nukes.
This is a bit playstyle dependent, but I don't like risk, so the Maddening enemy dex bonus giving quite a few enemies low percentage chances of critting my units is unwelcome. Blessing works as a bandaid for that, but also does a lot more than just protect against crits. The attack speed buffs also push enemy evasion up, so I frequently had to rely on linked support bonuses and breaker skills with accurate combat arts to avoid gambling, or failing that, to just reduce the risk, and I think the evasion is a bit too high for units that can't afford a dip into the archer class for hit +20 (especially vs. certain flying units).
I was somewhat disappointed with how boss movement worked out. Bosses being able to move actually seems to make Maddening easier in some situations, since you can lure them off the advantageous terrain, and so many of them are passive until you enter their range. Paralogue bosses generally tend to still be rooted in place. Then again, a chapter 4 where the death knight actively chases you would be a bit much.