That is not entirely true: for each Unicorn process there can a separate database connection for each site in the multisite. So 5 Unicorns, 1 Sidekiq and 5 sites = 30 Postgres server processes.
Some other disadvantages / things you need to be aware of with multisite:
- Sidekiq is shared and /sidekiq is only available on the master site
- All plugins are shared, there can not be separate plugin choices
- discourse.conf is shared so there can only be one mail server
- Some plugins are incompatible with multisite