The problem with unified mechanics is that everything feels the same. A lot of the fun in play for me comes—I think—from taking advantage of my character’s and my party’s strengths while minimizing the impact of their weaknesses. Which means there needs to be tradeoffs to be made. Brute force versus magic versus stealth. Strong fighting style versus agile fighting style versus ranged attacks. And so forth.
A subtext there is that a lot of games that tout their unified mechanic aren’t really all that unified. They still present tradeoffs. They just moved them about a bit. e.g. Despite arbitrarily unifying many (though far from all) things under the “roll d20, higher is better” regime, the d20 system ended up being a lot more complex than Expert D&D with it’s handful of different rolls.
The problem with “different things deserve different mechanics” is balance. It’s hard to well balance different mechanics. So, one set of tradeoffs often ends up too good against the others.
Like the way that two-weapon fighting rules in every edition of D&D that has had such rules have made it either too good to not choose or too weak to bother with.