Re:
My first MTB had them. They are pretty powerful, but that’s often negated by them bending the frame instead of stopping the wheel. They’re super fussy on straddle length and as the blocks wear you have to adjust the straddle and the blocks to keep them aligned or they eat your tyre. The later Shimano ones have a bridge to reduce the frame flex but it doesn’t solve the block and straddle problem.
Like others have commented, I remember sliding down Devil’s Dyke with a jammed rear wheel where the brake causes tons of mud to wedge between the wheel and the seat tube. The only fix is to stop and scoop it all out. Earlier frames with massively long chain stays are a little less prone to this, but that’s mostly a case of delaying the inevitable.
They do look cool, but in British riding conditions they’re really flawed. I wouldn’t use the adaptors, the seat stays will struggle with the extra force and you could ruin your frame.