First of all, no enemy spawns in your base if its closed,they can spawn near your base and enter an opened door or a place wich is not built.
They dont spawn in the workbench area but i guess they can spawn in your yard if you makd the fence at the edge of the workbench radius and maybe thats what people say.
Also, what mobs? Because during invasions they can break stuff.
Boars and dwarves will actively spawn when you are online, they will try to destroy your crops.
I lowered the ground by 1 tile near my farm and built a wall on top to double proff it and they didnt destroy my crop again.

I thibk people give more details like base layout, is it in the yard or base, biome and mob.
This may happen but so far in 90 hours and reading the forum daily i havent seen it