I'm not reducing, this is what recurring billing is. The relevant issues include cancellations, pro-rata refunds, card expiry, calendar calculations (always fun), etc. These are not rocket science.

Handling of tax has to happen on any payment and is not an issue specific to recurring billing/payments. It obviously brings in its own set of issues, the biggest of which is the multiplication of requirements if you want to comply with the rules of many countries, as you point out.