Assuming users have upgraded their wallets to handle P2TR outputs and trust the validation rules of P2TR, would there be any advantage of using P2WSH over P2TR?
As far as I can tell, P2TR is better than P2WSH in almost every way. I cannot think of a case in which it would be a significant advantage to use P2WSH over P2TR, except that P2WSH is already established.
P2TR outputs will stand out at first since there will be few of them. In the long term, the anonymity set of P2TR is likely to be larger since both single-sig and multisig will look the same until spent.
Engineering effort to adopt
Sending to P2TR addresses will require wallets to implement support for Bech32m. Wallets that want to receive and spend P2TR outputs will need to implement the Schnorr signing algorithm, and reengineer any
OP_CHECKMULTISIG based schemas with the op-codes available in Tapscript.