Far too often, teams get so excited about the possibility of automatically-driven topic pages that they wind up with an overly-complicated topic list. It seems so easy, what with the possibility of one template simply being driven by tags to generate lots of topics pages. Why can a complex topics list be a problem?
More difficult to tag well
Unless you are going to have dedicated librarians tagging your content consistently (or librarians managing the rules for doing automated tagging), the more complicated your taxonomy the more difficult it's going to be to tag well.
Less likely to be a good taxonomy
The larger your taxonomy, the less likely it's going to be consistent, correct, and not redundant.
More likely to be bureaucratic or overly-technical
Once you open the floodgates to a large taxonomy, you are far more likely to have it represent internal interests than those of your site visitor.
More difficult to have consistent topic page quality
The worst case is an empty topic page that a site visitor sees, but you can also have pages with such old content that it is less useful.
Harder to migrate
If you're making the topics list more complex in your target site during a migration, then you need to have a way of tagging it to this new list, and a complex list will be more complications for that migration.
As always, what the right level of complexity for your organization may be too complex or too simplified for another. After all, the whole reason for your migration may be more thorough topics pages. That said, keep in mind the downsides of a more complex taxonomy when designing your system to help ensure you don't end up with an unreasonable topics list.
