As someone who has had to evaluate product technical architectures for potential acquisition, groups.io gets a high rating from me. (Disclaimer: in my job, I would never make this statement without spending weeks or months reading code, talking to engineers, reviewing error logs, etc. I have had no such access-- nor would I want it. That was work!)
In terms of the product itself, I have no fears. I have also been impressed with the development process-- the way fixes and upgrades are evaluated and rolled out, security issues are addressed, etc. All this is to the good.
I must admit, I mutter a prayer for Mark Fletcher's health and safety on occasion. As far as I can see, groups.io is a one man operation, which explains a lot of its efficiency, attention to detail, and maybe some of its quirks. But single person products may not survive for long after that one person is run over by a truck or steps into an open manhole.
For me, groups.io's quality far overrides this concern, but if I were doing an evaluation, I would have to point out this vulnerability.
Best, Marv
Marvin Waschke