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Infer finds deeper infer-procedural bugs sometimes spanning multiple files. Linters, in contrast, typically implement simple syntactic checks that are local within one procedure. But they are valuable and Infer doesn't try to duplicate what they are good at. At Facebook we run both Infer and a collection of Android linters.
Findbugs can be useful too; it is more akin to linters.
On iOS there is the Clang Static analyzer. Infer does some things different, in particular reasoning that spans across multiple files. But CSA checks for more kinds of issues and is also more mature than Infer when it comes to iOS: we send big respect to CSA! Infer has only got started there recently. Really, these tools complement one another and it would even make sense to use both. Indeed, that's what we do inside FB!
About null dereferences, they are still a problem in ObjC: if you dereference a nil block it will crash, if you access an instance variable directly or try to pass nil to arrays or dictionaries it will crash. We try to find that kind of bugs.