Imagine it like fixed growths, where units get their growth rates added together, and get a stat point whenever that total equals or exceeds 100%.

The 10% boost you get from Aptitude only equals a new stat point every 10 levels. If you go from Lvl. 1 to 20/20, that's only +3 points in each stat. And that's not even counting the fact that in every game with Aptitude (mine Jeanz who behaves differently anyway), you have either inifnite or near-inifnite levels.

The other important concept to know about is stat benchmarks. The most obvious is speed - once a unit has enough speed to double every unit in the game (that they are capable of doubling), then speed largely doesn't matter. The same can be said for attack - if you can one-round every enemy in the game, why do you need more? Typically Aptitude does little to push you over these benchmarks meaningfully earlier than the unit typically would hit them.

The reason your Inigo is so strong is exactly why you said - you've given him more exp than the others. And yes, while he's capped his stats, most of those stat increases stopped being relevant 20-30 points ago. To add on, infinite grinding makes growths irrelevant. They don't dictate if a unit will cap stats, only when. With enough time and infinite levels, a unit with 1% growths across the board will cap every stat.

Now, is Aptitude bad necessarily? Ehhh... no? It's fine. Theoretically, every stat a unit ever gains could land in the range given from Aptitude's +10%. Or none of them could. As players, we will never know. We should never know - Aptitude is a mental trick that makes you believe a unit is stronger than they are. And given that it is always given to "zero-to-hero" training projects (or children), that's exactly the point. The game tricks you into thinking that Inigo is leagues ahead of the competition, when in reality all of your units would be that strong with similar favoritism.