Note sure if the tech side has changed much since about a year ago when I last used GUC's; I've found the process to be more reliable. It also seems there is better GUC inventory than I've seen in the past.

Some data points:

Aug 11, GUC availability AMS-SEA advertised. Used booking feature to apply to my booking. GUC cleared automatically within 5 minutes. I was able to check into the flight without issue.

AMS-SEA-AMS for mid Oct - early Nov. GUC availability advertised r/t. Applied to my partner's booking. This one did not seem to clear automatically which is likely due to it not being my booking. Used SMS support and they cleared both without hassle.

Also seeing D1 cash pricing having dropped quite a bit as well compared to a few months ago.