And/or (as far as I understand) when the output path is self-referential, so the out path cannot contain references to any path within out. Since fixed-output derivations use the hash of the contents (is content-addressable), it would not be possible to compute the hash in such cases.
An example of this is the MKL derivation. It has no dependencies and used to be a fixed-output derivation on Linux x86_64. However, later the derivation was modified to install pkg-config files. Since the .pc are installed to out and contain paths into out, it could not be a fixed-output derivation anymore.
(Of course, there are workarounds for such scenarios, by installing the .pc files in a separate derivation which depends on the fixed-output derivation.)