I have over 2 million posts on my forum and the sitemap generator works great.
Now I will advise you two things:
1. DO NOT let it crawl the calendar, the sitemap generator will not honor the allotted time frame set for the calendar, so I would turn the calendar off to guests and search bots.
2. Set up and run the cron job, it will generate the sitemap in less than an hour vs running the generator through the browser will take days or at least it did for me.
Just FYI, my site generates 6 sitemaps.