Well I didn't get into details but the implementation details like - performance - UX (user experience - efficiency of usage) can drastically impact the experience, for example, of people wanting to report issues and provide patches and that have to face something slow and hard to understand or use.
True. The success of GitHub and GitLab are largely based on their user friendliness in terms of pull requests, submitting issues, etc.
So I wouldn't consider implementation as something to ignore here, it's indeed the main issues reported and have a huge impact (I've seen people forfeit trying to post anything in the boost TRAC). The reason a lot of people would prefer to have everything in github is that the experience is almost "fluid" for any github user wanting to quickly patch something in any library
I think a detailed list of requirements in an issue tracker should be drawn up, perhaps in the wiki. Then for each proposed tool you can measure how well it meets each of those criteria, and whether or not it there are additional advantages or disadvantages.