You're right for the most part, but I hope that will change soon. I've been working on a new spam filter for a while, it's in testing on my mail server for a handful of customers, and quite a bit of stuff from constantcontact, sendgrid, Amazon, and even Gmail are getting caught by it.

The unfortunate thing is that every time there's a new trusted mailing service with a good reputation, spammers flock to it.

Not in this instance. They use your AWS SES account, which you could just as easily use from your own script so deliverability will be equal.

What would be a good way to measure that?

Your bounce rate and reasons.

A mail that goes into the spam folder does not bounce.