swapping crankset and chainline compatibility I'm looking into placing a power2max powermeter on my bike, which is full Ultegra equipped (R8000). Now, the chainline specs of the crankset from p2m are either 45mm or 46mm (depending on the brand of the cranks), whereas Shimano's FC-R8000 specs at 43.5mm. 

Are there any issues in swapping the cranksets, or am I facing some trouble or sketchy adjustments?
Yes, that's enough to create situations where you may have chain rub that can't be adjusted out in some additional gears on most road bikes. There are other factors, namely chainstay length - bikes with "pushed" chainstay lengths, i.e. 405mm-ish or less, are notably more finicky about this kind of thing.

How important this is depends on your riding habits. 

It sounds like you're looking at a power crank with a chainline that's either oriented more towards a road disc rear end (135mm as opposed to 130) or trying to do both with only one product.