Gmail is the only inbox provider that doesn’t offer a real feedback loop (you don’t actually know if a given email address marked you as spam when sending to gmail users). The FBL in Google postmaster tools is anonymized and unreliable at best.
So essentially, you never know if a Gmail user marked you as spam so you can stop sending to them. Gmail will just by default mark your emails as spam for that user going forward, without telling you. This means your spam complaint level will inevitably rise over time without you knowing why and what email addresses are causing the issue.
Unless Gmail actually starts providing a real FBL like other inbox providers, the hard spam limit is going to snowball into a nightmare for even the most conservative and legitimate senders.