Ultegra 6800 shifter with R8000 rear derailleur? I'd like to replace a Shimano 105 5800 11-28 cassette on one of my bikes (CAADX 2016) with a CS-HG800-11 11-34 to get a wider range of gears.

My rear derailleur is Ultegra RD-6800-SS, which can't take a cassette this big. I am thinking of replacing it with RD-R8000-GS (current generation of Ultegra, medium cage, takes up to 34t).

Are there any potential issues to be aware of with this combo (6800 shifter, R8000 derailleur, CS-HG800-11 cassette)?

P.S. The bike has a 44/34T cyclocross crankset. Both the front shifter and the derailleurs are Ultegra 6800 (11-speed). I plan to keep all of that as-is.
Should be OK. Both shifter and derailleur are 11 speed, only 1 generation apart. AFAIK Shimano did not change the cable pull or derailleur movement ratio.

All the other derailleur specs check out:

As you say, the R8000-GS goes up to a 34 tooth max sprocket.

Your front ring tooth difference (10) is within R8000-GS max of 16

Your total capacity required (34-11 + 44-34 =  33) is well within max of 39.