Okay, so I have been playing a T1 battle field mission in windstorm with both clockwork probe and urban crawler unlocked.
First time I rushed the enemy base and almost died, losing 285% out of 300% boosted health
But the victory gave me earth essence so I unlocked summon rhino.
Tried a battle mission again under the same circumstances, summoned a few rhinos and they smashed thru the enemy lines
Conclusion, rhinos are very good for this kind of task
However, I don't think they are over powered so please don't nerf them
By the way when I upgraded the rhino spell it looked exactly the same, I had to test to verify my suspicion that what I got was a tier 2 rhino instead of one of tier 1. But this should preferably be stated in the upgrade information for the spell, i.e. how the stats of the summoned rhinos change.