I think sitemaps is one of those area's where the complexity of multisite functionality becomes evident.
The main problem is that xenforo does not have sitemaps functionality itself and the sitemaps addon does not and will not support other addons. And everyone here runs addons.
But sitemaps functionality is essential. Without it or with erroneous indexation as explained above, your site is bound to take a large hit in ranking. There are 4 possibilities:
- the current sitemap addon will expand to support other addons. (not going to happen)
- xenforo includes it in a future release. (likely to happen sometime, but not anytime soon)
- xencentral multisite will include sitemaps functionality or release it as a paid addon.
- someone else releases a sitemap addons that supports all major addons and is easily extendable.
So it boils down to the question if xencentral is willing to resolve this vital issue.