Yeah. If you try out the Ultimaniac modes, you can see that the biggest issue in regards to difficulty is the aggression of enemies as opposed to the amount of damage Clive puts out. Enemies are really aggressive in Ultimaniac and they'll fuck you up really quick if you lose focus. Ranged and flying enemies especially. And if you've given any thought to your abilities, damage output shouldn't much of an issue either way.
I do think the lack of difficulty in the main (and more arguably Final Fantasy mode) difficulty was an attempt to compromise with the fans who weren't fully on board with the character action direction, but I think they had that covered with the accessibility rings and they probably should have upped the challenge a little bit.
Yeah, and Omega was noticeably more aggressive even on normal, and it threw a lot more AoEs and things to dodge.
They did play it safe in the base game, but it also seems they quickly took the feedback to heart, so I'm not worried about the future if they continue with this battle system.
Though, I think the bigger issue than the difficulty is how to get people to experiment with builds without compromising on the player expression the system allows for. There's a ton of fun experimentation you can do with this system but one of the recurring complaints I've seen is a lack of variety, because a bunch of people stick with the same abilities throughout the game because the game doesn't want to force a particular playstyle on the player.
Honestly I think the answer to this is simple.
Ignore them.
I don't think those that refused to experiment are really a significant group. It's common among the complaints, but it's best not to forget the game has an overwhelmingly positive reception. I don't think they need to risk messing up the design philosophy of player expression.
Also I think this is something that could naturally fix itself as they get the chance to develop more characters. Obviously a first character action game benefits massively from focusing on just one character, but if they iterate off it then giving the player three or four characters to customize(further building off the FFV jobs inspiration that lead to FFXVI's Eikon abilities and builds) and people will more often than not make difficulty builds for the characters.