If you are trying to validate an idea, then building it on a 3rd party service is fine, because your riskiest hypothesis is not, "We can run a real business."
But once you have validated the idea and are making a proper go of it, then yes, betting the whole of your company on some other highly risky company is generally a bad idea.
Basically, "never outsource something hard to replace" still applies, because early on you're happy to replace your entire business. If you have a few small customers and your vital 3rd-party service provider fails, then you say, "Oh well," accept that the customers are going to be pissed, and resign yourself to getting new customers. After you have rebuilt on a lower-risk platform, of course.
But once you have validated the idea and are making a proper go of it, then yes, betting the whole of your company on some other highly risky company is generally a bad idea.
Basically, "never outsource something hard to replace" still applies, because early on you're happy to replace your entire business. If you have a few small customers and your vital 3rd-party service provider fails, then you say, "Oh well," accept that the customers are going to be pissed, and resign yourself to getting new customers. After you have rebuilt on a lower-risk platform, of course.