I'll play Devil's advocate here...
The fact that there is a list of still outstanding issues shouldn't block a release at some point, otherwise you'll never GA a build. We will be the first ones to shout if Fortinet came back with "5.6.1 only going to be released when we solved all the problems, currently scheduled for mid-2018 if no one report further bugs".
I'm glad that Fortinet even publish that list of outstanding issues while other vendors simply don't. Any bug get prioritised and at some point when enough have been fixed a cut-off is decided, outstanding bug fixes are going to the next minor version branch and the cut-off build goes to QA. QA discovered bugs are prioritised again and either solved or passed to next version branch and at some point there is a decision to release. It's the normal process in software development.
Where I see an issue is the amount of regression bugs that appear with this release (and in general). Showing the difference between QA and real world utilisation.
I think one way forward that Fortinet could improve the issue is to do short beta cycle of minor versions to grab the most obvious/annoying bugs we are complaining about.
It would make the product looks a lot better, if the 2 or 3 most annoying issues would have been detected before release (top-summary, sections in policies, ...); they aren't (I believe) massive issues to fix and the fact they are so visible has a great impact on the confidence in the product, while realistically, beside those, this build seems far more solid and a big step forward to be honest.
We should all talk loud about stupidity od Fortinet's doings. Maybe then they will release firmware because it had reached stable state and was well tested, and not because the planned release dare was reached. This is a huge problem and it must change. Releasing firmware that has 5 or more pages of known issues in release notes is simply not the way to go, Fortinet. Treat your customers more seriously!