For anyone looking for a Yahoo Finance API alternative, try http://alphavantage.co/
(Disclosure: founder)
If the API provider is using a language+framework that can't easily handle lots of concurrent TCP connections, then yes, that could cause scaling issues, but... that's a choice.
There are plenty of ways to build scalable socket servers, and it's especially feasible in languages like Go and Elixir.
idk, it all depends on what you’re using the websockets for. If you have to send messages from one websocket client to another, things can get harder because those two websockets might not be connected to the same server.
edit: For those wondering what the price is it's on this page https://www.alphavantage.co/premium/ which is not very apparent from the front page.
It's akin ordering delivery from a store and having Instacart be the only option.
Most of the drama was misinformed speculation. A bit was suspicion that the order fulfiller was untrustworthy (because the reason they are willing to provide the service is that they know you will make bad trades, so what happens if you make a good trade that makes them lose money?)