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I have never found Xoops to be an obstacle to getting good SEO results. In my experience, most modules are correctly spidered provided (as you point out) articles have well constructed title and meta descriptions.
Use of heading tags is also important, but most WYSIWYG editors allow these tags, plus you can hack the Xoops DHTML editor to recognise H1, H2 etc., although it would be nice to have this as a standard feature.
The only real chore is debugging modules to make them W3C valid. I'm sure that validation is less significant for high-traffic sites, but a valid site adds another brownie point to the search engine scorecard. The htmlValidator plugin for Firefox is really useful (this page currently has 110 errors, but it increases with each post).
I do use URL re-writing on a couple of non-Xoops sites. It doesn't cause a problem but I cannot say for sure whether it actually makes any difference to SEO. From what I understand, Google don't encourage the use of re-written URL's and I found conflicting opinions as to the benefits of using re-writes or Pathinfo. I would certainly be interested to see the results of a test, provided it was truly independent and not part of some SEO company's hype.
At the end of the day, although there are guidelines for module developers, there are no enforced requirements. So, a module might validate, it might not. It might build titles and meta tags dynamically, or it might use the same tag as the site pref on every page.
It might work, or....
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