I would urge caution when going down the path of suggesting that any one thing is so paramount as to be worthy of intense concentration to the exclusion of other things.
Instrument scan pattern and rate is very important if you are hand flying but the mark of a good pilot is his/her ability to keep it all nicely ticking along, assigning appropriate priorities on an as-required basis throughout a flight.
I suspect that a good pilot has a characteristically short concentration span which reduces the likelihood of fixation. This particular problem can get one into all sorts of difficulties.
One should use the autopilot and other automatics in an appropriate manner to reduce manipulative and cognitive workload.
If handflying, make sure that trim is maintained and that thrust is set symmetrically.
Naturally, if it is a nasty, bumpy day, everything in the difficulty department escalates.