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 not supported yet. It is unknown what will happen when trying to access a modded GUI.
Entities are not supported yet. It is unknown what will happen when trying to display a modded entity.
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. But when using custom recipe types (custom serializer), your recipe won't display in the clients recipe book.
Custom packets won't crash the client. But they do spam the console. In the future PolyMc will block any custom packets.