Basically, they offered a lot for free then gradually, with each new version, removed more and more features from the free plan to the point where you can't even choose a country, let alone server. As a free user, you only get option to connect to random (not necessary good) server and you can only use the service for web browsing; torrent is completely blocked as well. This is why I don't like them; basically luring and scamming users.
I get that they are a company, they have to earn money, nothing in life is free, yada, yada, yada. But man... it's just wrong to leave your free users in the dark. They are still your users after all. You might not earn money on them, but they have power to refer you to their friends and family, who might fell in love with the product and purchase it. Not all users need access to all countries/servers, and not everyone needs port forwarding or some other additional features.
This is why I tend to stick only to popular DNS services as they are less likely to cause issues. I'd only use Cloudflare's 1.1.1.1, AdGuard Public DNS and Google Public DNS. I refuse to use anything else as you never know what issues might arise.
From what I understand, AdGuard when unable to resolve domain on their own, they ask Cloudflare and Google to resolve domains for them. All while protecting your privacy, not forwarding your details. If neither can resolve domain, then the website is unreachable. NextDNS, on the other hand, doesn't have a backup and if they can't resolve domain, it's automatically unreachable.