It does come with some drawbacks, even though I understand none of them. "Inability for the payee to specify their preferred min_final_cltv_expiry"; why is true for keysend and not for invoice? "Loss of being able to use the preimage and invoice signature as proof-of-payment"; again, which part is different and I don't get the pre-image? Isn't it a core part of Lightning to get the pre-image? Isn't there a payment hash involved?

One drawback I expected to see but didn't is privacy. To send with keysend, you need to reveal your node's public key. With invoices, that's not necessary.