If you so desire the information is out there, and there are blogs and so forth on the internet that link to the details.
However i will summarize. Mastadon has taken a staunch position against racism and nazi ideas, which is fine. However they will go after those who are not so militant in this en devour, even if they do not support the aforementioned ideas, there's a lot of shaming that goes on if you don't all decide to block the same instances for example. However they have, many times over, allowed certain, borderline illegal (usually linking to illegal) content to be freely accessible on said platforms in the early days (for like a good 6+ months) of mastadon. After a lot of complaints they still did very little to nothing to stop that behavior. This is the source of my hate for mastadon and those instances. They have shown in the past that they are not consistent in what they block, they do not allow others to have agency and still federate with them, and they have turned a blind eye to the proliferation of child abuse. Gab has a hard block coded into mastadon, you can not federate with them unless you actively modify the code; however, other instances that are completely untenable are not blocked in the same manner.
It was for this reason that the great "split" happened, about a year or two-ish ago. Where a lot of mastadon instances no longer federate with large portions of the fediverse. It's caused a weird bubble. I just will never forget the twisted concept of, "if you are racist, or like Hitler we can't have you" while on the other hand, leaving a large (believed intentional) gap for illicit materials that personally, it breaks my heart to see that kind of degeneracy. On our instances we do not block political ideologies, however when it comes to drawn or real child abuse, we have never allowed it, and will never allow it. It is one of the greatest evils of our time.
EDIT: this is my opinion from what i saw back in the early days, they could have changed, i don't chance it, just know when reading that this was a long time ago in internet years and a lot of those people aren't as active as they used to be. I hope they have improved the community, but i will always hold this grudge.