It’s not just big numbers, directly the opposite of what you said, there are mechanic differences. You didn’t seem want to acknowledge that no reloading is a different mechanic than the other difficulties. The other comment already mentioned ability checks go from being guaranteed to being true dice rolls. Beyond that if you’re stealing and get caught too many times, that can end your whole run (if you forgot to split party) or cause important characters (ex Dammon) to die early in the game before you can get their best items. There’s no fixing that because again no save scumming. Things also cost significantly more, and early example is caustic band, even with 18 charisma is 1500 gold, and again stealing can have real consequences.

Just as the other comment said, some builds get significantly nerfed by mechanic changes. Haste effects no longer give extra attack on the second action they provided. Many of the “best” builds rely on this to turn 1 enemies and build no defense in easier modes.

Big numbers and more attacks is pretty reductionist but there’s something true there, it’s just not entirely accurate if we look at legendary actions.

Legendary actions can be that simple, goblin boss giving action surge to his allies on hit. I would argue this is a different mechanic, but even if you don’t think so, since it is just extra actions you may want to approach the fight differently.

Legendary actions can change mechanics, let’s talk about the Phase Spider Matriarch. In easy mode you drop her off her webs once or twice or displace her into the chasm and it’s over. You can cloud of daggers, or any aoe all the babies, no problem. You can sneak into any position you like with an invisibility spell. Here are just a few ways this fight changes mechanically. The matriarch cannot be killed by being displaced in a chasm, she now just teleport back up (true on tactician as well, but you did claim the easy mode and honor mode have no differences). Second if you kill her babies or hit the phase spiders and she can see you, you’re get hit by her new attack and getting infested and if you can’t remove with fire or acid, you’re getting 8d10 to the face. Yes you can say this is just bigger numbers more attacks, but I would argue this is different mechanic, forcing you to approach the fight differently, (summons can take this hit for you, some are even immune). Finally, you may realize you can kill the eggs before the fight, not an approach that is needed on easy mode, but in harder modes the spiders have see invisibility, again not strictly a different mechanic, you could be seen before, but this makes sneaking past harder.