Status
PolyMc provides good support for this. You can decently use a mod with these with a vanilla client, as far as that is possible.
These won't crash the client. You can still play with these but you might not be able to fully use them.
PolyMc doesn't support this yet. They might crash the client, they might not.
95% of tested items work fully with a vanilla client. For some advanced items they might not display properly though.
Blocks are being worked on. Full blocks already work completely. Transparent full blocks kind of work. Other blocks will just display as stone.
GUIs are being worked on. Any modded gui will currently display as a furnace gui and are not useable.
Entities are not supported yet. They will probably display as a pig.
Custom sounds won't crash the client and won't do anything. PolyMc will provide support for custom sounds soon.
Custom recipes will work in a crafting table or other crafting station.
Custom packets won't crash the client. But they do spam the console. In the future PolyMc will block any custom packets.