Well, it weaponises CSAM. If its put on chain, it could initiate a re-org. During a re-org there's opportunity for a double spend attack.
Discussion
Valid concern, but it seems related to the activation strategy (reactive path instead of a simply proactive one) more than to the data carrier limitations proposed by the BIP.
I personally feel that a good way of thinking about these limitations is: if these had been in place since the very first time Satoshi released Bitcoin, would Bitcoin had been unnecessarily limited and would it have failed? (the answer can also be yes)