People have been complaining about Rees bunkers since I have been on here, and that's been over 20 years now. I get it, especially now that styles have changed.
For some perspective, my take on the dislike is simple.....Rees bunkers are often, 1) too large, and 2) have too much size for their shape.
While you can make the case that on an open site like TP, large bunkers fit in against the open skies better than on a fully wooded course where dense trees provide a smaller space. That said, I have studied bunkers of all types for years.
It just seems to me that larger bunkers need a lot more shape and variety of shape than those at TP to be truly attractive. Look at Mac's bunker at 10 ANGC. It is a masterpiece of every turf and sand lobe being at and of a different size and angle. Sometimes, when being built with big dozers, there is a real tendency from the size of the machine to even all those out to about the width of a dozer blade.
In regard to greenside bunkers, typically (with exceptions) I find that when bunkers are further outside the green than it's width, they dominate the scene. As in, a green 60 feet wide probably should have the outside edges of bunkers on either side no more than 60 feet away from the green surface. I also find that the turf "noses" should usually be less than 25 foot from the green edge. While obviously personal opinion, I think some deep thinker or another, who has studied art composition, could probably craft a scientific argument as to why that is true, LOL. At TP, many of those bunkers are large enough to become visually disassociated from the putting surface. I think Rees and Bobby both got their idea on "scale" from their father, and rarely deviated from that style. Understandable, they were mentored by one of the greats, and how much were they going to question their basic style?
As to shape, the first time I read the phrase "Rees pieces" was here, and I think it came from Tommy N. Derogatory, yes, but full points for being clever. When a bunker is round, it needs to be smaller than he typically built them. I think the idea came back from Scotland, either via Pete Dye, or the ASGCA trip to Scotland in 1980. Or, earlier, the Robert Bruce Harris oval or clamshell bunkers, which never looked good. Many ASGCA members brought back the idea of pot bunkers, but then enlarged them to accommodate modern maintenance, or maybe the then scale of American golf courses. Round bunkers also generally look better when clustered with a few cousins......but, when Rees tried that with his (IMHO) oversized round bunkers, a clever moniker was born, LOL.
Most of his bunkers at TP are more shapely, but I think the corners, capes, and bays were tuned to the radius of a bank mower (or sand pro) for ease of maintenance, which is not uncommon on public courses. Bunker rakes turn in a diameter of about 16-18 feet now (used to be 20+ feet) and bank mowers are still stuck around 25 foot diameters, near as I can tell, but I think a few can mow tighter. Using those dimensions (and exceeding them) tends to soften out the shapes and make bunkers bigger.
But, there are times when that IS the most logical way to design a bunker that fits all needs of a public course. Whether it is for a US Open venue is another story.
There is perhaps also the question of what the USGA thinks of shaggy bunker edges that are popular now? (Although, they didn't have a lot of trouble with his brother's Chambers Bay) Ress has had many opportunities to discuss the USGA philosophy on how bunkers ought to present and play in their major championship, and we have to think it is USGA directed to some degree.
Certainly, competitors probably hate the idea of missing by 5 feet and being in the long grass, or missing by 8-10 feet and being stuck in some craggy and unplayable edge, when missing by 15 puts you in the relatively easy bottom of a sand bunker. Add in the longstanding USGA requirement for narrow fw and flanking bunkers (which actually worked pretty well, with many of them coming into play, especially on the canyon holes, where they were the safety outlet. A separate point, but I wonder how Rory felt about being up against the back lip of a bunker and shanking it? Yes, rub of the green, or sand, but I have heard lots of discussion from players over the years about any situation where they "have no shot" from a bunker, usually when playing from a downhill lie, and especially when up against a back edge.
Anyway, short version, they don't have enough shape to please the eye for their size. Or, in layman's terms, they are a bit, shall we say, "blah".
But, visuals aren't the only consideration, although, in the TV age, I think the probably should be, LOL. I have heard stories of the TPC course directing green backing bunkers (like those Pete Dye wraparounds) so they look good from the cameras behind the green. In one case, they directed a waterfall be built facing away from golfers but facing towards likely TV tower locations, etc.