[source] + 指出 + ,+ [clause]
[person] + 指出 + [noun]
正如 + [source] + 所指出的 + ,+ [clause]
指出 introduces what a person, report or study says, and presents it as a fact the writer accepts. The source is the subject, and the content follows as a clause, usually after a comma.
With a noun object it is "point out" in the everyday sense: showing someone a mistake or a problem.
正如…所指出的 brings in a source to back the writer's own point, "as X has pointed out".
需要指出的是 and 值得指出的是 flag a caveat the reader shouldn't miss.
指出 treats the content as established; 认为 reports it as someone's view. A news report writes 研究指出 when it accepts the finding and 有人认为 when it only relays an opinion. The citation uses of 认为 have their own point.
指出 is not pointing at a physical thing – that is 指着. And it already contains "say", so 说 doesn't follow it.