On BrainKing, Embassy Chess which uses the same setup as Grand, is more
popular and the smaller 10x8 board seems to work better for the 2 added
super pieces. Grand Chess' main distinction is the extra row behind just
for the rooks which is probably not as interesting as the creator thinks it
is. Generally, those rooks just back up stronger pieces in front of them,
or they are exchanged with other rooks on open files.
It's more interesting in normal Chess where a rook tries to get on an open
file by clearing other pieces out of the back rank than it is like this.
In short, the freedom of the rooks and the extra space for the King to move
around lead to less tension in the game which is not a good thing. 10x8
games with those 2 pieces like Janus Chess and Embassy (Bird/Capablanca)
play better on 10x8.