Russian TTL metered finder questions

Any pitfalls with using one of these on a 500 C/M? Are there any specific models and/or suppliers to avoid? Thanks, Bill Barber

I use one on my Rollei SL66, forget which brand but it's one I see used on Hassys too. Works fine but is much dimmer than when you look with a good WLF. Thing is I shoot mostly b&w and hardly use it either using a handheld spot or incident meter, or off the top of me' head.