[heavyset_go]

Assuming your adversary is a state with access to certificates, a malicious or compromised exit node could lead to your de-anonymization and access to information you may want to keep confidential or hidden.

Your connection to an onion service is end-to-end encrypted and authenticated, as well, which means no MitM can trick you or sniff your traffic.

[lxgr]

Ah, no reliance on the web PKI is a very good point (especially if their site also accepts document drops etc) I didn't consider, thank you!