Yes, Robosats is all on lightning, but they use hodl invoices to conduct the bitcoin escrow. If I'm not mistaken, Robosats also use a centralize coordinator node to hold and control these hodl invoices; so it's not an actual peer to peer exchange, putting users at rug-pull risk and also putting Robosats at regulatory risk (the centralized coordinator is an easy target and can be shut down). I think they also use a centralize order book, but I'm not 100% sure about that.

Compared to that, Bisq is as decentralized as possible. There is no centralize order book or centralize server to do escrow. Everything is done peer to peer, other than dispute resolution, which is done by Bisq mediators and arbitrators. But even during dispute resolution, the mediators can only suggest a resolution, and the arbitrators can only arbitrate which trading party to get whatever percentage of the escrowed coins; they cannot steal the coins. In other words, the rug-pull risk and regulatory risk are minimized, as they keep everything as decentralize as possible and try to do everything peer to peer (no centralize target to attack).

In the end, it comes down to trade-off. Better UI/UX+lower fees vs risk minimized+robust.