For what it is worth. I was always under the impression that most of those companies that are using these networks known for 'harassing' where just ignorant. I used to do business with the 'idiots' of Tucows/opensrs, trying to explain to them that it is not really wise to send password reset emails via the same mail servers that their 'cheap clients' are using for spamming.
However I just got email of a company medialab.co using this mailgun network. Turns out they had problems with getting blacklisted and that is why they moved there. So I tend to change my position, that it is quite legitimate to rate these networks as being bad by default. Maybe most clients just move there because they are sending shit. And now they can use the excuse that someone else caused the bad reputation.
