I have had no problem with this ui at all the original ui that was in this light wasn’t that good but from what I’m seeing this ui is similar to the U22 since I was told the U21 is discontinued. The only problem is that my anti-glare coating is gone off my lens and I need to find a place to recoat it.
I've seen a few poor implementations of press&hold mode switching because the are too slow. There's some better quicker ones as of late, think'n maybe the new SP70 for example (not 100% sure though). But if the press&hold mode changes are done quick, it's soooo better. True you can't go backwards typically, but there's no reason they can't implement a timed delay toggle like we do in ramping, then that solves that problem. Not sure if Anduril works like that with mode levels (not ramping), but that would be cool if it did.
I absolutely hate waiting for a light to turn OFF - not for me. Again, it's a matter of how long it takes - many of these UI's are designed way too slow for my tastes, and I'm old, but I'd debate say'n I'm slow 
Ohhh btw - I find the Anduril delay for turning the light OFF unacceptably slow - many don't, I know, but I do. I've asked and sounds like TK has no interest to fix it.
I have been watching flashlight reviews on YouTube nitecore has a few with the ramping you like Tom the acebeam k70 has a dial on it to do ramping from low to turbo and a strobe it’s a tail cap button that if you want an extreme thrower check the reviews. K70 High Intensity Flashlight|AceBeam® Official Store | Flashlights, Tactical Lights
Sofirn, and 1 or 2 others are adopting the click ON/OFF thing, but many manufacturers are inconsistent as well, and could depend on old vs. new models, and maybe even who is writing the firmware. They probably use multiple sources, and maybe can't always dictate the UI - lots of contractors/providers, etc...
Always interesting to open up a light and see a competitor's name on the PCB's - wut? Oh yea... :facepalm:
Can you fit a 21700 battery to this flashlight?
No it’s either 26650 or 18650 with the sleeve that comes with it. 21700 will be too tall.
but the extra 5mm can not be done by compressing the springs?
No the springs are max compress with the 26650 I know because my flat top is pushed in from the springs.