The free tier of NextDNS is limited to 300K requests per month. One can easily hit that limit, especially if done on the whole home network. Once the limit has been hit, ads will start showing for the rest of the month unless you subscribe or have multiple accounts.
Read their
Privacy Policy. At the end of the day, it's up to you to decide whether you trust them or not.
There are other similar alternatives as well, such as AdGuard DNS and Control D. I currently use the latter on my home network for the other features it provides. Anyone who connects to my network will see no ads, even in YouTube and Disney+, without having to install anything.