Yikes. This is a brave position to put yourself in; kudos. Is it tough to deal with deliverability, esp w/o control over what clients send out...?
It is certainly trickier than I expected at the onset of starting the project (which is true of pretty much everything related to the email space), but not overly onerous. Most of my risk lies less in content and more in recipient, and I pour most of my efforts into subscriber validation during onboarding & activation.
Does that track with your experience?
I think it'd be a really cool blog post if you had the time to just share what you see.