In the specific case of MooSend though, please coordinate with them, since they did the integration for us.
Yes, I understand. The problem is that these "service suppliers" (not only MooSend) are not getting much interest from Zen Cart users (since the user-base has gone down). So they kinda abandoned these updates and development.

I installed that module a couple of months ago and I had to disable it because it was not compatible (it made my cart inoperable, it will not let you checkout). There was no error logs so I couldn't tell them what exactly was happening. Since there are not a lot of people asking and/or using it, they would not proceed to update the module.

The more the merrier. When I did this work, MailChimp was the only game in town for SMBs; now there are lots of options.
I'd love to see more integration work.
I know, and that's why I mentioned MooSend because there's also an integration for ZC but nobody talks about it.