websockets and sse are a big headache to manage at scale, especially backend, requires special observability, if not implemented really carefully on mobile devices its a nightmare to debug on frontend side

devices switch off network or slow down etc,... for battery conservation, or when you don't explicitly do the I/O using a dedicated API for it.

new connection setup is a costly operation, the server has to store the state somewhere and when this stateful layer faces any issue, clients keep retrying and timing out. forever stuck on performing this costly operation. it's not like there is an easy way to control the throughput and slowly put the load on database

reliability wise long polling is the best one IME, if event based flow is really important, even then its better to have a 2 layer backend, where frontend does long polling on the 1st layer which then subscribes to websockets to the 2nd layer backend. much better control in terms of reliability