Grum is overrated
I am happy almost every time I see Grum on the other side of the battlefield. He is so slow, with 1 speed I have almost never experienced him hitting any of my monsters and even if his speed is increased a bit he is still a very easy stun target.
An example:
https://splinterlands.com/?p=battle&id=sl_7bacf39ea5e554b2dee23d47d1d45156&ref=truls
When you use 11 mana on a monster it has to be more than a huge speed bump.
I have tried to play him with Kitty and it makes him much better, but he is still a perfect stun target.
When you have the reverse speed ruleset he is amazing of course, but there are many cards that are great in one ruleset.
From a brawl battle:
https://splinterlands.com?p=battle&id=sm_vfQn76DmQRy1lUAth6DQ&ref=truls
0
0
0.000
I think he works better in the second position.
Yes, in any ruleset were back line monsters can attack he can be good if combined with Kitty or his speed is increased.
Reverse speed or Kitty and higher mana matches. I'm not sure how clutch or rather 'win more' he will become.