I think they are all Friggin' awesome knives. Debating details and then saying one is better because of them when each is unique in design is like asking which one of your children you'd keep if you had to choose.
Just because they have different traits does not make them good or bad. It makes them more ideally-suited to an individual based on perceived need, that certain details to one person will offer advantages over other details. That's my take on the matter.
A 21 seems slimmer and lighter in design than the 25. But the 21 bushing system seems on paper and in use to make it pretty much identical to the larger pivot on the 25 so details equal out there. Is either one weak? NO!
So now the 25 has no bushing so it's inferior? No way. My 25 ROCKS without a bushing. And it doesn't need no "steenking" Locktite. Once you tighten up the pivot with both wrenches to the sweet spot, the whole thing rotates in the handle.
You don't have to crank it down until it never moves and it will not loosen up, at least mine never has after hundreds of opening and closing cycles playing with it. And if it needed a little Locktite? I'd put a little Locktite on it and it would be sorted!
It comes with it just in case. It is not mandatory to be used. Doesn't make it a worse knife if it were to be used. That is a very trivial item. A Raptor and all other aircraft use safety wire. Does that make a Raptor a POS? Maybe you should ask one of the 12 other aircraft it took out single handedly if it is an inferior airplane. It is not in spite of being safety-wired in places also.
CRK makes a variety of knives for a variety of people to do one thing. Be used as a tool to cut things that need to be cut at one point in time or another. How you get there to do that depends on your own liking. Not a good or bad choice, just a choice!
Your choice does not make mine a bad one, nor mine, yours. I'm glad we have so many choices, but with one common denominator. Awesome and consistent quality in each and every one, be it 21, 25, Umnumzaan, Insingo, Tanto, whatever details. They are all very high quality tools, crafted with exacting detail. Each and every one. Consistently.
Each one would (should) last each one of us a lifetime and if it wears out, a new blade can be inserted and it will continue on doing its job.
Steve at GFeller leather, who makes CRK leather items, was on his third blade in his 21 when they shot that two-part video in 2006. I'll bet he's on his fourth blade by now, 8 years later. And I'll also bet that whether or not it was a 21, 25 or Umnumzaan would not matter. Any one of them would still be plugging away daily.
I own one of each, but no Tantos. Doesn't make me think a Tanto is a POS just because I don't own one. I just like the appearance of the other blade styles. I'll bet in a push cut situation with that tip it would be awesome, too.
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