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Bo-Ming, I had a Metabones Leica R to FE adapter and quickly returned it. While we are talking about the EF to FE here, I'll share my thoughts.
1. A specialized wrench is needed every time one wants to attach/remove the tripod adapter
2. The fit at the lens to adapter coupling was very tight, beyond my appetite for repeated mounting/dismounting. I don't know squat about the machining and mechanics of such a thing, nor properties of which metal slips past which and which wears first, etc. etc. but I'd never needed that level of effort for any bayonet system, ever.
For those who use the EF-FE adapter, how much force is needed to turn the lens in mounting it to the adapter, compared to mounting an EF lens on an EF body?