5
Google does not have any problems indexing XOOPS sites.
As of ranking by Google, without a pagetitle hack, everything from XOOPS is shown as
sitename - module name
This does not help in ranking. All relevant information is listed under "site name - module name" and is treated as secondary by Google ranking system. Google has given pagetitle more weight in ranking.
I hope the XOOPS core developer would consider caching pagetitle by header.php. The pagetitle hack becomes useless once the modules cache has been enabled.