With the game doing all the calculations and rolling for you, 3.5e mechanics wouldn't be any more difficult for a player to grasp.
I can’t comment on 5e in table-top, nor digital form, but automatic calculations and rolling make DND really difficult to grasp in digital setting, especially a convoluted system like 3.5e or pathfinder. I am over 130h in Pathfinder and I am still at a loss as to how half of the mechanics works - precisely because I don’t get to interact with them directly. They just happen. Why, how, what influences the roll... wiki only knows.
Back in a day I beat BG2 with little to no clue as to how mechanics work, but due to its simplicity it was intuitive. With 3.5e it doesn’t work like that, and players can screw themselves over far easier. Free multiclassing was a nightmare first time I interacted with it in NWN2, and I feel there is only that much convolution one can handle when micromanaging a party of characters.
BG3, being turn based, might actually handle a more complex system better, but without playing it, I can’t comment if 5e works or doesn’t work for me.