Are there some out there that are tough even for grand masters? Do the top players in the world still study tactics?

Most/all of the good guys I know do training exercises to keep their powers
of concentration sharp.

Jacob Aagaard found that loop hole in the Fred Reinfeld book which
featured on a recent whilst 'topping up.'

My observation is that good players who have left the game and returned
tend to find their tactical ability has suffered (they miss things) whilst their
positional sense has never deserted them.