I bought a 32GB Sandisk 2000x 300/s UHS-II Extreme Pro (cost £52) and compared it with my new 64GB Sandisk 170/s UHS-I Extreme Pro (cost £22) on my 90D. Under the same conditions, I let it go for the full burst and then another 5 shots. With the UHS-I the burst was 22 full RAW and then about a shot every 0.5s, and then 15s to write fully to disk. For the UHS-II, it was basically the same a burst of 23 shots, then a similar speed of 5 shots in about 2s , and 15s again to write full to disk. If I try continuous shooting, there is the burst of 22 shots, 7-8 slow ones in about 3s, then a series of short bursts with pauses in between. Importantly, the camera keeps working with these short bursts while the writing to disk continues and doesn't seize up - as space in the memory becomes available, the camera will fire.
Do I think the the UHS-II is worth the extra cost? Not for the way I use the camera, but others might think so.