Some alternatives like Subscribestar are already offering the option of using cryptocurrency, so the biggest problem (going from cryptocurrency to whatever the creator uses) arguably has a solution.
No ... the biggest problem is making it user friendly, secure and amiable to recurring payments.
The last one is perhaps the hardest nut to crack, subcribestar doesn't even try AFAICS ... it's just for one time payments. First off unless your cryptocurrency account is linked to your bank account, it can't have the same reliability as a recurring credit card payment for the content provider. So either you allow automatic topping off from the credit card (and invite a whole lot of overhead from the credit card companies, charge back potential and all) or to mitigate it you could require minimum balances before allowing new payments/subscriptions. Even the technical implementation of the recurring payment is not trivial, you will pretty much have to implement it on the Ethereum blockchain and instead of a normal address use a smart contract for the customer deposit.
For user friendliness you probably want to partner with Coinbase so you can handle everything with a single signup, people don't want to go through multiple sites to get everything to work (yes they do that with PayPal and Patreon, but Coinbase isn't PayPal).
One alternative for the security of custom hardware might be to implement the smart contract so transactions can only be done to other smart contracts of your own design and keeping the money in escrow for a while, with the ability to reverse the transaction within that time. You massively increase the burden on your company by allowing charge backs though. Those are pretty much the choices though, if you place all the burden on the customer to caveat emptor then all transactions must be absolutely securely signed off by him ... if you want to take short cuts and allow signing transactions on mobile phones, or worse computers, you pretty much have to have some kind of charge back mechanism for when some zero day lands and empty everyone's accounts.
It's a huge amount of work to get something which theoretically would not instantly turn off a normal user, or turn him off after some inevitable security lapse if you don't design it securely. That's before you even start marketing and see if they are willing to use it.
PS. I'm not sure subscribestar will be able to continue for much longer ... without PayPal and Stripe they have to either deal directly with credit card companies now (huge overhead) or with more traditional payment providers (big fees, and for anything porn related huge fees).