Cards are usually not banned for power level reasons, and the last time it happened was Lurrus Of The Dream Den prior to the nerf to the companion mechanic
As it costs only two mana and has haste, you'll likely be able to trigger this ability multiple times in the early game, stealing important top-deck pieces from your opponents
[20] After the Aurora, merrows became more fishlike
The original Ultimatum cycle had some powerful effects, but it was Cruel Ultimatum that left a lasting impressionon the game, on my opponents, even on professional Magic
Its a fair amount of text to parse, but in short: when you trigger Gimli to fight, he will be indestructible, so dont worry about it
This gets weaker the further you stray from mono-green, but I can live with only spending 4 or 5 of the 6 mana my dork provided