Big vendors like Google, Facebook and many iab vendors make Consent a mandatory as part of their terms.
If non-compliant, data subjects, NGO’s, consumer organizations and competitors have the right to sue you.
As a Controller, you have to decide on which legal basis you rely for processing personal data.
Data authorities all over Europe have started to publish statements on how and where Consent applies.
When you use certain Google products, like DoubleClick, Adsense or Firebase, Google enforces their EU user consent policy.
In conclusion, you cannot use the Google services unless you have obtained a proper informed consent of your users – BEFORE you start tracking.
There are more than 400 vendors supporting the iab Consent & Transparency Framework, e.g. Criteo, Adition or Taboola. Certain vendors will only serve your ads, if you can transfer the consent technically, which means you will need a CMP that supports the technical format of the iab Framework.
Filing a complaint is very easy and possible online on the website of the responsible data authority.