The only real “downside” is when you want to have shiny surfaces.
As said above, the you would have to set the roughness all zeero.
So all the ON micro facetts are coplanar and non occluding.
The lambert model would do an equivalent appearance with a fewer instructions and resources.
So, if you have no rough surfaces in your level, go with lambert
If you have mostly rough surfaces, then go for ON.
If its a mix of both, then decide:
Lambert : “cheap”, but does noty a good job on rough materials
ON: More expensive. Renders rough surfaces good. Renders lambertian look with more costs than native lambert.
Personally, I think the additional rendering costs are worth the effort.