I've never seen an official announcement from Betfury that they have a mirror site, and I don't remember them mentioning it. I would be wary of clicking on these links and entering my casino password, as these links could be phishing.
Then that simply means that you've not really been active on this thread.
Well, to be honest actually, I've never really come across any official statement from betfury team saying anything about their mirror sites, or them having a mirror site, but alot has been discussed on this board as it patterns to this, and I have actually used two betfury mirror sites to access my betfury account, and this I did successfully with any issues or any thing to be suspicious about.
Many platforms, specially casinos (from my personal experience) have mirror links through which the website can be accessed safely and securely incase if and when the main domain is not available or online.
To be safe using mirror links, all you have to do is to make sure the link have been used by other gamblers and they confirm that it's safe to use.
Really after using stake mirror site a couple of time, I am well familiar with how mirror works now, as best alternative to the main site Incase of troubles to access the main site, but the major fear for me sometimes is if I am going to encounter some troubles in carrying out some activities on the mirror site as against how the the experience with the main site has been, although I have used the mirror site to play games and also make a withdrawal without any stress.
Anything also is if there is chances that one can get a fake mirror sites all with attempt to use big platforms name to create a fake mirror sites with the aims of stealing visitor's details and other informations, this is why I agree with what you said concerning reading other gambler references of the mirror sites link before using them, it save you a whole lot of stress and risk of getting into the wrong hand and site's.