I have a 12.9 2018 iPad Pro, I love it. It's still pretty fast despite being more than four years old, they really future-proofed this thing with the A12X. Two issues though, first one (as others have mentioned) is the RAM. 4gb is still okay, it's not terrible but it won't be the best with multi-tasking. There's also the problem with the iPad being easily bent. I have mines bent unfortunately, so you gotta be careful with it or add a good case to protect it. I regret not using a case which is why mines got bent.
Other than those two issues, it's a pretty good iPad.