According to Polymorph's description,
The new form
can be any beast whose challenge rating is equal to or
less than the target’s (or the target’s level, if it doesn't
have a challenge rating).
For example, a Tyrannosaurus Rex (MM, p81) is a beast with CR 8, 136HP, multi-attack, +10 attack modifier and a lot of damage.
Can a Wizard, at level 8, cast Polymorph on another PC (also at level 8) and turn him into a Tyrannosaurus Rex?
This sounds awfully broken. CR is usually measured against party level. A CR 8 enemy is meant to be a decent threat to a party of 4-5 PCs of level 8. We could find a ratio here, where each player accounted for 2CR (sort of…). But Polymorph lets one of the players actually equally face an enemy that could challenge the entire party.
I realize that there aren't any stronger beasts than the T-Rex, so the power-level doesn't grow, but it still feels like a spell that can easily break encounters. E.g., big boss fight incoming, some big baddie with a couple of helpers. The Wizards turns the out-of-spell-slots Bard into a T-Rex and then hides behind a pillar and inside a Minor Illusion Box. The T-Bard wrecks havoc among enemies while the party supports him and the Wizard is smoking his pipe hoping no one remembers he exists.