My 2 cents (based on 20 years of overclocking, system building, and competitive benching and hands on with a few Z390 boards):
The REAL issue with the Asus VRM setup on in Maximus XI line is/was misleading advertising. Asus has now removed most of the advertising material that claimed the boards were 8 phase (twin 8 phase). But out of the gate it was a bad idea to imply the Maximus XI line had 8 phases. I suspect they did so in order to compete with other brands that actually offered more phases. But no matter the reason, it was a bad idea and looked like purposeful deception.
The secondary issue is that for the price point, a fat 4 phase implementation is not competitive with the other brands. The fat 4 phase actually performs well, but at the ROG high end, it is not competitive with other offerings from say Gigabyte. It appears Asus skimped on the design, offering adequate VRM performance, but not living up to a premium offering ROG is known for. Not good.
However, that all said, this is where the truth needs to be clear. The 4+2 VRM on the Hero/Code/Formula are FAT phases with parallel components. This design FAR exceeds a normal 4+2 setup. With fewer phases to switch on and off, the fat 4 phase does run warmer and less efficient than boards with more phases to share the load. However, the temps on the Maximum line never reach dangerous levels. Heck, I can run my 9900k at 5.1 GHZ AVX consuming 228 watts on my Hero XI for 2 hours and my VRMs never exceed 68c in my case with typical case cooling. 68C is warm, but not alarming. But they are about 10c warmer than competing boards with more phases.
The Extreme board is 5+2. Again these are FAT 5 phases with doubled up components running in parallel. I guarantee you that the VRM on the Extreme will not be a limiting factor. Look around the net at world record overclocks on the 9900k and you will see most are held by the ROG boards.
What Asus SHOULD have done is say outright that they were moving in a different direction with their VRM implementation. There is more to a quality board than the number of phases (yes, I actually dared to say this lol). I've seem some real junk 8 phase boards out there. All else being equal, more phases IS better. But the quality of the components of each phase is also important. Also important to a quality board is PCB design (affects how cool the back of the board runs), memory tracing, a good UEFI, and more. Asus should have just said that they were focusing on other aspects of the board and defended their fat 4 phase instead of being worried about the marketing hype of how many phases the board has.
No doubt Asus save some cost on their design. But I can tell you from hands on experience, the VRMs perform well. Maybe a bit warmer, but it's not a horrific design that should be avoided at all costs. Should it be better for the price point? Yes. But when you look at all the boards and brands, you can see what boards are achieving the best overclocks on both the CPU and the RAM. You can see where Asus design shines. So you need to evaluate the entire offering. In my case, I am not willing to trade the best UEFI, great PCB design, and superior memory overclocking in order to get a board with more VRM phases. Other may make a different decision. And that is ok too. But I would not run away from the Extreme solely on the number of phases issue. The Maximus line VRMs are definitely adequate. They just pale in comparison to some of the competition. But the Maximus XI shines in other ways as well.