Seems like more of a "to each his own" kind of thing with every post I see. Are we really talking about such significant differences in performance when it comes down to it? From the reviews I've seen, nope. You can keep your large chunk of ugly metal if it works well for you, and I'll keep my sleek looking AIO, because it's certainly working just fine for me.
Actually, that was the point I was making about (what seems to be) the obscure EVGA ACX cooler. It's hardly bigger than a 212 EVO, and none of those air-coolers need be deployed with a fan attached to obtain maximum CFM through the fins to a point where no further increase in cooling can be measured. That leaves the noise factor, which can also be effectively mitigated, and I had shown last year how that can be done.
The ACX will trump an H110i by about 3C degrees, and it can do so without sufficient RPM to cause any more noise than you would get with a dual-fan H110 also pushed to its maximum cooling effectiveness.
But it's also easier to achieve those results with an AiO. At that point, with those sorts of narrow margins, then the issues boil down to minor aesthetics, fan choices, and extra tedium.
Put it another way. I set up my sig-rig in 2011 with an NH-D14. At that time, it was not clear whether the AiO's were made with reliable pumps, a disaster or failure had been reported here and there, and I was able to get great results beyond what the reviews would show. I have no incentive to change it out.
For $55, the smaller ACX cooler was 6C more effective than the D14 generally, and my attentions to it made for 12C more effective than the D14 as tested in the reviews. There was a 3C margin over an H110 -- 65C at full IBT "Maximum" load stress. This second example was as much a matter of curiosity for me. So at that point, choosing air versus Aio would only be a matter of aesthetic difference.
The biggest hassle with the AiOs arises from fitting them to this or that case, choices of intake versus exhaust and so forth. All cases have a rear exhaust fan, so the right heatpipe tower makes an easy fit, even for some "aesthetic" drawback. At that point, you can weigh the shortfall of heatpipes from AiOs without special attention, and the match to AiO cooling with "special attention."
The simplest decision would be to use an AiO, but with an H80i, it becomes "more effective" with the same special attention or tedium you would apply to the air-cooler single-tower. For instance, you might still have trouble with a 240mm AiO radiator in a C70 case, which equals the trouble of fretting over installation of an H80 for stellar performance.
So with those sorts of margins, it actually is an "each to his own" fuzzy area of choices.
I'd say that AiOs -- without more than following the instructions for stock installation -- are better at the margin than heatpipes in stock installation. They're just not that much better.
They can't be, because they're marketed as an easy-installation choice, so they have to fit a lot of different cases. The radiators are only going to be so big and so thick.
If one chose the path of ducted heatpipes, top-end fans and acoustic "mitigation" with them, you're at a different 'starting point" than someone who just bought a good heatpipe cooler and stuck it to their mobo. To get out ahead of the pack using AiO's, the answer is custom-water, custom-chosen radiators to fit certain cases, maybe dual pumps and dual radiators where you can fit them.