'how could you know how little or much effect the GDPR has'
The OP has a point... So like marriage and your favorite sports team etc, low-expectations are best here. For example, take this blunt cautionary note from Max Schrems of NOYB fame:
"Tech companies will likely do the maths on GDPR sanctions to see which problematic features are so profitable that they can afford to keep them running - or at least eat a one-time fine as an experiment in testing the EU"