In that case C#/EF/SQL Server is what that app was made in. his was like 6 years ago, admittedly, but it didn't geel as if it's really treated as a first class citizen. Everything's in ints in example code, you have to fight the auto-code generators a bit, etc. So in my experience it's never anywhere near as seamless as the int support.
But it's not just the support that's such the problem. You're testing, you need to switch category, you can't just change a 1 to a 2. You have to go find what random uuid the categories had added to it. You can't just go into the DB and add a new line, you have to open a UUID generator. You can't just quickly add a foreign key relationship, you have to look up the UUID. And a ton of other little annoyances.
Actually, categories are an excellent example of something that shouldn't be a UUID, they're actually supposed to be discover-able.
I think my present project has UUIDs on the user, company, invoice and payments tables, but still ints as the primary key. Everything else isn't worth it. There's a merchant table, but again, they're all supposed to be discover-able (and aren't editable by the merchant themselves).
I also generally implement controller level security that checks access to the root object being returned by default, so I can't really make a mistake exposing an unauthorised object. There's an occasional controller where I've made a conscious decision not to implement that level, generally actions that allow both authenticated and unauthenticated users (e.g. viewing merchants or categories).