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XOOPS' lack of URL rewriting/SEO URLs is a major stumbling block for it to be honest.
Without spending hours checking, I believe XOOPS (and its clones) is the *only* CMS without this feature. It seriously means that I cannot use it in a commercial environment. At least not without hours of work hacking about with code.
It doesn't seem like much, but it's that one feature that is letting it down, and people expect this with their sites these days.
However, as for URL rewriting in Joomla - it is there, but it is still at times hit and miss. Even in some articles, it might just display the article ID and not the stub/alias. Then if you set up a menu link to an article, the current rewritten link breaks, and vice versa.
I've not tried any comment modules in Joomla, so I don't know how they compare - but again, I don't think there's anything wrong with the comment system in XOOPS, which seems to work well.