I'm currently transitioning a paid app of mine to free+subscription, and RevenueCat has made it incredibly easy to set up so I don't have to worry about managing purchases and validating them, which is allowing me to launch the update a lot faster than expected.
We're relying on RC's infrastructure to handle the subscriptions in WeatherKit. To me, RevenueCat is an extra layer of security that people can purchase subscriptions, use the app and have a good experience. That layer is invisible and it makes it incredibly easy to use & implement. If you want to build a subscription business - start with RevenueCat.
P.S. From the moment we decided to use RC, to the moment where we were confident that this works perfectly, it took us 4 hours.
We've used RevenueCat to implement our subscriptions for our Design+Code iOS app and it was a blessing for us. We saved a ton of time, resources and received valuable advice/expertise. The system takes care of all the edge cases, which is so crucial for not losing customers. Building your own in-app purchases feature is super hard to get right, so this is hugely recommended!