I think it’s a bit of a blanket statement though that multichain always necessitates a bridge to another chain – the design of IBC (Interblockchain Communication Protocol) allows for a verification of consensus transcript and state of another chain without any of the governance related issues you mentioned that bespoke bridges introduce. So, I guess it would be more accurate to say that the the security uncertainties with multichain have to do with bridge implementations which are unreliable or dependent on governance mechanisms.
In this vein, I think cross-communication protocols for L2s as they become more widely used will face a similar dilemma as L1 multichain approaches – LazyLedger/Celestia would use IBC also to communicate between their rollup chains, but it seems to be a larger discussion that is still evolving.