I think that a significant reason why Grizzly feels good is because Amph feels good. Scallop/Buoy/Bulkhead-based armies with Lobster/Limpet support frequently put me in a strong position on my lane, then Grizzly arrives to deliver the
coup de grace.
Scallop and Buoy are not only strong, they also prompt the enemy to build an army that can engage from long range and is unlikely to want to dive you - which is, in most cases, exactly what Grizzly likes to fight.
Obvious exceptions here are Lance and Sniper, which are quite effective against both the cheaper Amph units and Grizzly. Handling Lance and Sniper is as much a tactical problem as it is an army-composition problem: most units can beat Lance/Sniper
if they can engineer a favourable engagement. Finding a solution to this tactical problem is left as an exercise for the reader. (Lobster and/or area-cloakers should give you a good start.)
On some level this also applies to Grizzly. Plenty of units can beat Grizzly if they get a favourable engagement... but if the Grizzly gets to take a few shots then retreat and repair, or an attempt to dive the Grizzly runs into Scallops and Limpets, the fight is bad or catastrophic.
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Compare to Ravager: I have heard some noise lately about people saying Ravager is overpowered, and I could imagine that in a vacuum Ravager is a better unit than Grizzly - but Rover does a much worse job of setting Ravager up to succeed IMO.