Did some testing with the new MLAG feature and wrote a blog post on it.

Have you considered e.g. using a GRE tunnel interface as the peer-port to be able to ensure IP multi-path redundancy in your test topology? Thereby avoiding relying on the physical peer interface not being interrupted during normal operations. (e.g. cable unplugged by accident, optic/port hw failure)

Some networks will use MLAG for geo distant (<50km) devices with e.g. L2 Network-to-Network Interconnects (e.g. 2x1x10 GE, 1x10 GE NNI per router, in an active-standby set-up). An example for this could be two service providers, where provider a uses provider b's access net to reach customers. With the set-up being provider b backhauls all traffic as L2 circuits to a geo-redundant L2 NNI between provider a and b's networks in two different datacentres.

[admin@CSW-01] > interface/bridge/mlag/set peer-port=gre-tunnel1
invalid value for argument peer-port:
    input does not match any value of interface
    input does not match any value of interface-bond

One thing I noticed is that the documentation says "The MLAG requires enabled STP or RSTP protocol" - does MLAG not work with MSTP?

I just tried using MSTP on the Bridge the MLAG ports are on, it definitely does not support that version of STP.