I've been on UC (edit: Universal Credit) a while and when I was first on it would lose messages (on both my side and the jobctr officials, and it has frustrated them because I talk to them Edit: and they have their own usability frustrations with it, with little or no way to feed them back up), log you out even before timing out, lose appointments or have them be set but not show up for you, and more.
There's plenty more I could say but I'll just leave something which sums it up. At the top it says "BETA This is a new service - your feedback will help us to improve it". Guess what, it's disabled so you cannot give feedback. It always has been disabled.
Visually pretty good but in terms of usability, not so good. Pretty sure no proper testing was done on it, though it has been getting a lot better recently.