Many web-driven apps are affected by silly cookie settings. Vendors of crappy-apps like "Norton Internet Security" make a living out of selling their silly apps, luring users into a false sense of security because they block something as innocent as cookies, while they allow users to continue using the unsecure default browser with unsecure default settings in place. Does this mean that Coppermine (or any other web-driven app that relies on cookie authentification) is bad? No. It means that users don't care and blame the web-driven app, just because they don't understand the technology and the silly twist the security discussion lead by Symantec and others has made. They even made you believe that the web-driven app is to blame instead of the client PC and the obscure settings there.
Just because there are several threads of users who have similar issues as you doesn't mean that Coppermine is to blame - remember that this board contains thousands of postings; you will find many nearly identical reports for any issue users did ever have, yet for the vast majority, everything is working as expected. The threads you refer to do not prove anything.
Remove the wheels from your car and then blame the construction workers who built the road that your car is not running...
After all, Coppermine's target audience are webmasters who understand how a webserver and a browser work and interfere. If you don't understand the mechanisms that make the whole www tick, don't draw false conclusions. If you're not capable to set your client up to make it work as expected, then maybe coppermine is not the right app for you.
Joachim