A few years ago, I'd have said that the uncapped stat scaling was a good thing. Now I'm not so sure.
My reconsideration has nothing to do with any sort of "realistic" gradient for how well humans (and demihumans) can develop their physical and mental abilities...heck, except for Strength (with the encumbrance rules) and Intelligence (using the old "1 point of INT is equal to 10 points of IQ," though I've heard arguments both for and against that shorthand) there's now way to objectively measure what the ability scores represent anyway.
What's brought me down in regards to unlimited ability score advancement is how it seems to play towards power-creep. I had to really lean on my current gaming group to get them to adopt rolling their ability scores (4d6 drop the lowest) rather than point-buy in our games. The reason I wanted them to change? Because I was tired of seeing every (non-Multiple-Attribute-Dependent) class start out with a 20 in their primary ability score (point-buy an 18, raise to 20 with their +2 racial modifier) at freaking first level.
Yes, this hit them in other stats, but the characters had specialized so well that it often didn't matter. The alchemist with a 20 Int and a 7 Strength was going to stay the heck away from melee to begin with (usually spider climbing onto the ceiling), and it's hard for other characters to used ranged attacks at him when the Strength 20 barbarian (then raging for Strength 24, and later 26) is in their midst dealing obscene amounts of damage.
None of these are unsurmountable problems, and I certainly was able to challenge the party; what I didn't like was that this was the new normal. Characters who started with an 18 in their primary stat were considered "imperfect, but workable," while those with a 16 were as underpowered as anyway was willing to go - anything less was unacceptable.
Admittedly, some of that comes from differing player expectations. I'm of the opinion that heroes start as ordinary folks who rise up to do great things. My players believe that heroes are heroes because they start out "a cut above" the ordinary people.
Even so, however, I think that uncapped stats are part of the issue with higher and higher ability score bonuses - stat-boosting spells and items, ability points every fourth level, inherent bonuses, and on and on. It doesn't help that they're mandated now, thanks to spellcasters needing them for their higher-level spells.
Personally, I'd much prefer to leave the system uncapped (as a closed system means that you need to cover the entire spectrum of power in a finite limit - how much stronger is Tarrasque's Strength of 25 than a storm giant's strength of 24? etc.) and eliminate virtually all stat boosts that aren't racial bonuses or specialized class abilities (e.g. a barbarian's rage).
Capping a system brings it's own problems; I'd much rather do with ways that are used to promote upward-stat-mobility.