Backwards-compatible Payjoin usage is still synchronous and so requires both parties to be online at the same time
Discussion
Is that the only downside in V1?
From BIP 77:
“However, BIP 78's requirements for Payjoin Version 1 have proven to be an obstacle to adoption. Version 1 receivers must host a secured public-facing HTTP server. Mobile and web environments limit the ability to fulfil such a requirement. Version 1 also requires synchronous communication. Both sender and receiver must be online simultaneously. Wallet developers regard these requirements as barriers to Payjoin adoption.”