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Really depends on the use case. When you don't need the extra features of the AWS load balancer+API gateway over, say, a relatively straight forward nginx/caddy installation. The time to learn the AWS equivalents over off the shelf OSS is pretty similar. Why not just use the OSS all things considered?

There are significant downsides to a lot of the vendor offerings, they seem like a simpler solution but they are plenty complicated in their own right. Particularly since they are probably built to accommodate varied and complex cases that you don't really need. And rarely is this paired away under some special dialog for power users, often you are expected to just know everything up front.