Builderb0y might be able to shine a little light on why Sinytra Connector is problematic for running Fabric mods on Forge, but from the gist that I know, Fabric mods that make use of mixins does not play well if at all when run through Sinytra Connector. And, unfortunately for you, many Fabric mods heavily make use of mixins. Only a select few doesn't use mixins, those are fully compatible with Sinytra Connector.