Summary:
These are great replacement/backup batteries from a known reputable brand. An original SINGLE Sony battery will cost you around $87. With K&F, you're getting 2 batteries and a way to charge both of them at the same time for less than that. The only thing to complain about is that the charging speed is pretty slow.

Pros:
K&F is a known and reputable brand in photography. Although there are definitely cheaper batteries from Chinese vendors, you really don't want to risk gear worth thousands of dollars on unbranded unknown dangerous things like batteries. Just look at any reviews of those cheap batteries and you'll see people complaining about swelling, loss of battery life, even some expanding and getting stuck inside the camera body.

You can connect with USB C or Micro USB rather than a proprietary connector. This allows you to use a portable battery bank to charge the batteries if you're out on the go rather than needing to be home and near a wall outlet.

Cons:
Although convenient to charge two batteries at once, the charger is really slow. It took around 5 hours to charge both batteries to full. The charger is definitely made for convenience rather than speed. You can just drop your batteries in overnight and they'll be ready the next day. I shoot relatively infrequently so I don't need to charge too often and so it doesn't bother me as much as it might a professional or dedicated hobbyist. Don't expect to charge them and use them the same day if they're at 0.