Yes, it is difficult to do without a lot of testing. I've covered a few over the course of time, but nothing which includes a lot of input from multiple vendors. I only have tested Swiftech, Bitspower and EK blocks...I think that's all. Some of those are monoblocks for specific boards.
I did a brief look through. Most of the Threadripper blocks I saw were very expensive and didn't look all that impressive. One was $200, all copper, but honestly looked like the cheapest one to make. Thermaltake had one that just looked like what the early ones were, adapted blocks from AM3/AM4/Intel.
Then there are the notorious Enermax AIO that clog up.
Yes, but there aren't that many Threadripper blocks that actually cover the area of the heatspreader. Enermax's AIO does, but it doesn't have good longevity. Though you can always drain it when you get it and refill it with coolant or distilled water.
Most just look like a more standard sized block with a bracket to mount to the board. Covers most of the center of the CPU, but misses the edges completely.