Aside from aesthetics, the main knock on threadless stems is height adjustability.

Aesthetics? I think quills look better on a bike with classic steel or titanium tubes. I think threadless look better on more modern, larger aluminum or carbon fiber tubes.

For height, it really is a non-issue. You can get as much height adjustment pretty darn easily with a threadless stem -let's say, 1-2 inches - which is all one would generally be looking for with a quill stem anyway.

Don't cut the steer tube, leave it at the maximum length allowed by the manufacturer (usually ~70-80 mm from top of bearing to top of stem = 30-40 mm spacers + 30-40mm stem). Use stem angle and spacer placement to get the bars where you want them.

Higher bars: put all the spacers below the stem (if not there already) and/or flip the stem to angle the stem upwards and/or swap to a more upward angled stem to get the bars up.

Lower bars: flip the stem downwards, or change to a less angled stem flipped up or down ... and/or put a spacer or two above the stem - limited to compression plug guidelines. Heck, get a different compression plug that requires spacers above the stem if needed - they're cheap and easy too.

Difference between a steep angled stem downwards vs. upwards - probably more than an inch. You also have two intermediate steps if you're willing to invest $35 in a second stem.

Add another 1/2 inch for a 10mm stem above vs. below the stem.

So there's no reason you couldn't have good incremental adjustability - easily equaling a quill stem - without a lot of hassle.

My guess is that those of us who bike enough, and know enough about fit to be concerned with this are maybe the type who use maximum spacers, and they're all normally below the stem anyway. So the height adjustment would be a simple flip of the stem or a quick swap to the "spring" or "summer" stem.

Is this as easy as just raising or lowering a quill stem? Maybe not, unless your quill stem is seized, but certainly not more than 10 minutes of easy DIY effort with normal tools, a couple of times a year. And you won't end up scratching your beautiful, possibly vintage, cold forged quill stem. I won't even get into situations where you want to change bars.

The only limitation might be one's sense of aesthetics. For some reason, among the aesthetics police of road bikes, spacers are a no-no, as are steeply angled stems. That's just dumb. The threadless system is designed specifically for that sort of adjustability.