If you read their reviews on Amazon you can see they are rather mixed. I have had at least a couple of false positives on them and I'm starting to doubt the lines they gave me this cycle because Fertility Friend put me at 5dpo when I saw them Monday (I put me at 6dpo then but that's still incredibly early for lines like they gave and tbh there isn't a huge chance I ovulated earlier). There are also loads of people saying they consistently gave them false negatives, or that they only started showing up much later when they claim you could see a positive 6 days before missed af. Of course this will depend on level of hCG in the urine, but some of these people aren't seeing clear positives until after af has been missed, and at this point there isn't a huge chance levels are going to be quite that low since they're meant to be 10mIU.
I think they're ok if you're a poas addict, as they're cheap to buy plenty of, but I wouldn't assume no line means you're not pregnant unless af shows up, and I wouldn't assume lines meant pregnancy unless something else shows lines too