NextDNS is a commercial entity founded by a Netflix employee who is working on a Netflix CDN. Do the NextDNS terms of use address the potential for data sharing between the two entities.
Running NextDNS has costs. Given the absence of fees for using NextDNS, it has a commercial interest in collecting information about users. Like other third party DNS providers (middlemen), e.g., Google or Cisco/OpenDNS, NextDNS supports ENDS Client-Subnet. This extension has zero value in terms of ad-blocking and privacy and arguably should be "off" by default unless the user asks for it.
PiHole is non-commercial project AFAIK, although they have registered a trademark.
Third party DNS caches will always be inferior to DIY in respect of certain issues such as ad-blocking, privacy, security, reliablity, etc. (I am a DIY-er and when third party DNS has an outage, the applications I use are still able to use the internet without any problems because I have zero reliance of third party DNS providers.) When using third party DNS these factors are outside the user's control. Users cannot tell third party DNS providers what to do, nor can they execute quality control, they can only accept what is offered to them. Of course, third party DNS will always be superior in terms of convenience and perhaps "features". I personally do not need all of the "features" offered by third party DNS, but I cannot speak for other users.
The user's "choice" between DIY and third party DNS depends on what is important to the user and what the user is capable of doing herself. When the user is not capable of running DNS software herself, then DIY is removed from consideration and the "choice" is simply between one third party provider or another. The user has very little control in that situation.
When it comes to DNS, for me nothing beats having control. For me, "control", not convenience, is the best feature. I prefer whitelist to blocklist. Every user is different.