You can still debug in Upp. Once the debugger reach the breakpoint just wait for a while. Don't press F10 or anything. After 10-20 seconds the variables under 'Auto' or 'Local' will come up and you can start stepping through your program with F10.
It somehow seems that the communication between Upp and GDB is very slow. I don't know what's the reason for it. It used to be ok with older versions of Upp with MinGW.
It's a bit annoying, but still can do some debugging.