So I haven't been very active around here lately, I'm too busy using XY
Anyways, here's a few things I noticed and remember :

Create some UDC. Assign a KS to it. Remove the UDC. Now you are in a weird spot, where pressing the KS still triggers the (removed) KS, as well as being "locked".
On CKS trying to assign it reference to the removed UDC, so obviously it can't be removed or re-assigned. I haven't tried with saving & restarting at that point (I'm too scared to loose a KS

) so I just recreated the UDC, went to CKS which didn't list the KS but showed it on the list once clicked, to I could removed it & then removed the UDC.
Not that I'm not sure it's linked to the new "Assign KS" button, which I used, but... that's it. I'm out of words on that one, sorry.

I think it might be good to have File Associations to support Environement Variables as well. In fact the icons does support it, so that makes it even weired they're not actually supported. I guess.

There's a weird thing going on with parameters. When I use parameters on Open or Open With, either on UDC or Catalog, they're not sent. Not unless there's a < somewhere on the command line.
So when I don't use <variables> it never works

I do not have a clear pattern to reproduce it, but twice already when adding an UDC XY crashed. Yes, I had added/removed some during the current session I believe, but I do not know what triggers it.
All I know is, first time I pressed Ins, heard a bad beeping noise (like some XY tried to popup some error message, maybe more than once whic resulted in the big crash, not sure. Anywas,) and then 100% CPU use and XY not responding.
I had to kill it.
Right now, I had pressed the New button and it just beeped again and crashed, disappeared.
Not much to help you I know, but it was always when adding a new UDC, that's for sure.