I have the Ultegra R8000 compact crankset & front derailleur on my Canyon Endurace CF SL. The bike originally came with a 5800 series compact crankset and derailleur, so the R8000 was an upgrade. I also have the 6800 Ultegra compact crankset on two road bikes with caliper brakes and 130mm rear road hubs.
Each of these work well, but I would strongly encourage the use of the R8000 Ultegra on any gravel bike and on any bike with disc brakes. The 6800 Ultegra compact cranksets when installed on a 142 wide rear hub will cause the chain to contact the large chainring when the derailleurs select the small chainring and small cog.
It has less to do with shifting performance, per se. I think that it has to do with ensuring that the chain adequately clears the big ring when the chain is on the small ring and towards the outside of the cassette. Regardless, I'd go with the R8000 on a disc brake gravel bike.
With the R8000’s, Shimano redesigned chainring tooth profiling, increases clearance between chain and leading teeth to providing improved compatibility with frames featuring the disc brakes and wider dropout spacing.
Well, I learned that the chainline issue, when combining a 142mm thru-axle disk brake frame with the Ultegra 6800 crankset, is no joke. When I first attempted to use a 6800 Ultegra compact on my Canyon Endurace, I was unable to use the two smaller cogs while on the inner chainring.
The chain was rubbing against the derailleur cage when using the 11t cog of the cassette and the 34t chainring of the compact Ultegra 6800 crankset. The next cog, a 12t, could have been used but the chain had almost zero clearance from the cage. If the front derailleur was adjusted to eliminate any chain and derailleur cage interference while using the 11t cog, the chain would then contact the pins and the leading teeth on the 50t chainring and jump between the two chainrings.
I eventually sourced a R8000 crankset and matching front derailleur for the Canyon Endurace. This combination allows the use of all 22 chainring and cassette cog combinations without any interference of any kind.