My concern with this line is that both Fiala and Kempe can be difficult linemates at times. They're both prone to making frustrating and stupid decisions with the puck.
I agree discipline is a major issue with both Fiala and Kempe. Fiala, when controlled, can be a good playmaker and Kempe has greatly improved his shot, so I think there's good synergy there.
With Byfield, he has good reach, speed, and puck recovery ability. So when Fiala does his no-look spin around pass, Byfield is better equipped at trying to recover. He's also more consistent at digging into the boards to complement Kempe's inconsistency in battles.
There are definitely concerns, but I think Byfield can compensate for their weaknesses while still being put in a position to score.