@roveer google and cloudflare only filter bad stuff.. Or at least they say, but only in rare cases with google, clouldflare is more open in saying hey we block bad stuff.. But lets say dns service A filters bad site X, but service B does not.. But since you don't know where your forwarder might ask at any given point in time, are you protected from bad site X or not?
So what is the advantage of their filtering? And once its looked up once, all your clients will get that answer if they ask for it, etc..
The filtering or not filtering would come more into play if you were using say a blocking dns service to block stuff that you want blocked, like adult related stuff, etc. etc.. While your 2nd service does not. Opendns or Umbrella or Adblock sort of services.
Many people choose to use say quad9 because they list blocking bad sites as one of their advantages.. Cloudflare kind of says the same, while google says hey only in really bad cases, etc..
The point is these example services are not all filtering the same way, even if only bad. But that they filter at all - and you wouldn't know which one might get asked at any given time - kind of really throws all of their filtering out the window.. So if your going to use different services, but you don't know if service A and B would block the same thing - you can not be sure you would ever be protected by such a service.
Maybe you forward to these services because of their bad site blocking, but also maybe they block stuff because government says hey block this.. The point is such services are not all going to filter the same way, but if you ask more than one will you or will you not actually be protected, or filtered from something you want to get to, etc.
Same thing goes for if they do dnssec or not - most all the major players do, unless you specific use a special IP they list, etc. But this is another thing that can be different if you don't actually know who your asking when you forward.
While I am not a fan of forwarding - many people do, and hey that is their choice.. My only point is if your going to forward.. No matter who you forward you should get the same answer.. If not it can be very problematic trying to track down some weirdness with some some specific dns query.
