We have been selling them for a while and there have been a few issues on NA cars - its a pretty small percentage to be fair but it happens.
Also Berk do not guarantee they will pass a UK emissions test - so in the case where it doesn't then there is no warranty claim with berk
With Berk you only get 12 months warranty.
With Cobra you get two years warranty.
If you want a really solid HFC then I would recommend Fast Intentions - they are pricey and they are a long lead time
It all depends on preference I guess, I had Berk HFCs on my Zed no problem