While I wil never subscribe to any app as a matter of principle, ultra is a completely voluntary extra. The app has most functionality implemented already, and all lemmy functions have been said to be planned as part of the base app.
There is no membership required to use the app. Even the subscription isn't required
I gladly paid for mine, as I did with Sync for Reddit before. Got extreme value out of this, will get extreme value out of the 20 € I paid here. So, yes. "Memberships". If they are worth the price.