I only do hobby gamedev, so I don't know what I'm talking about, but when I think of a tool that's "too complex" to release, I imagine not just a conceptually difficult thing, but a dangerous thing. For example, the editor lets you shoot yourself in the foot with a bunch of a esoteric considerations that don't really make sense, but the internal people know how to work around them more or less, like making sure that there's a dummy Bar for every Foo on the map, and that they are named in some crazy way, and that the name doesn't contain the "%" symbol, and you have to do that all manually and if you don't the host computer hard locks. Internally they solve this by hard-won intution about how to use the tool, plus a huge and thorough testing scheme.
I think that's the key -- if the tool allows you to harm the user's computer in some way, then it's not a tool you can really release without polishing a lot more, to prevent the failure modes. And it's particularly bad if it's hard not to mess up.
