The complexity was more in our customers application logic then in Stripe. Stripe understandably enforces quite a few sensible limits that 'normal' usage probably wouldn't hit. For us the 2 biggies were:

* There is a (undocumented?) limit of 25 subscriptions per customer. Our client sells a la carte services and some large customers have quite a bit more then 25 things. Subscription items partially solve this EXCEPT everything in a subscription must bill on the same-cycle and...

* Our client had a requirement to maintain the existing billing cycles for all customers (and enforce specific start dates for new subscription).

These two limits meant we had to bundle existing services into Subscriptions during the migration and use the "trial period" feature (as a "start_date") to force the billing cycle for each bundle. The worst case customer had I believe 18 different billing cycles for their services in the end so we got lucky. (we also had to figure out a way to name the bundles in the UI and the CC statement descriptor etc).

In the end this saves on transaction fees and reduces the clutter on their CC statements but cost quite a bit of development effort to do and makes the code managing "subscription" starts and stops a bit complex.