Same here on multiple machines. I think you might run into problems though, when the following happens:
You save a file while running your favorite OS, OSa, then let the change propagate across your cluster. Then you change (say “save”) that file again while still on OSa, immediately shutdown that OS before syncthingA got a chance to notice or even propagate the change. Now you start OSb, syncthingB here sees a change on the cluster, but also a change to the local file, so you will get a conflict on that file.
I had that happening a couple of times, until I couldn’t bear it anymore and now I also share my database across operating systems thanks to a tip by @AudriusButkevicius: You can open an index.db folder on the shared partition and then symlink both ~/.config/syncthing/index-v0.11.0.db and the corresponding windows folder to that folder. If you’re sharing a fw hundred Gigs of Data that also saves space on your OS-partitions
I also share the certificates, so that I can “reduce” the number of devices in my cluster. Note, however, that I don’t share the config, since they use different paths.