A downside of building your own custom API is the fact that figuring out a perfect API strategy that will be really functional is hard at times, and here we will try and see what the disadvantages of building your own REST API would be. RESTful approaches in development are pretty hard given the fact that a client-side and server-side interaction is required and that a single set of requirements is required for both sides, including testing and actually building the client-side and server-side parts. There's also the fact that given the plentitude of available, modern technologies, it's pretty tiresome to create an ideal development cycle and process. Interface negotiation would have to happen between both parties, and that consumes efforts from both sides, which makes it harder to see the project finish line, yielding a lack of satisfaction for all parties.
Money and time consumption is not the only hassle that is very expensive and time consuming, there is also...