My advice is to only depend on commodities, not services. a commodity is something so generic and interchangeable that you can hardly distinguish between competitors. In this way, every external thing you rely on is a potential single-point-of-failure, but can be easily fixed.

If the thing youre depending on is unique and essential to your business, you're taking a risk because your success depends on their continued success and if that service goes down you go down with it because it cant be easily replaced.

TL;DR You dont have to reinvent the wheel, but for all of the off-the-shelf products you use make sure there are plenty you can replace it with at any time.