At the moment, jammr only supports MIDI Clock, which is basically tempo.
Tempo works perfect when syncing short-term things, e.g., guitar FX tails. I feed the MIDI from jammr to my guitar FX to keep things in sync & it works perfectly.
No matter how perfect your gear is, a bunch of individual devices following tempo over any significant amount of time will drift apart. It's just life. No two pieces of gear stay perfectly in sync independently of each other.
That's why MIDI also supports commands like MIDI Start/Stop/Continue. Another option is to have MIDI send the SPP, Song Position Pointer. Different gear supports different protocols. (MTC, Midi Time Code, is the equivalent to SMPTE, which is for film/video & needs things like frame rates… Not a jammr need, I think…)
But I think if you could get jammr to xmit SPP on the MIDI bus, or at least xmit a MIDI Start at the start of each interval, that should be sufficient to slave most external gear, and in fact, to have different folks syncing different MIDI gear. As long as your MIDI gear functions well as a MIDI slave (they all say they do, but some don't really, they all want to be the master…) and honors MIDI Start &/or SPP.
Hence this enhancement request:
https://forum.jammr.net/topic/2188/