I've made a career out of inheriting other peoples whacky setups and supporting them (as well as fixing them) and almost always its documentation that has prevented the client getting anywhere.
I personally dont care if the docs are crap because usually the first thing I do is update / actually write the docs to make them usable.
For a lot of techs though crap documentation is a deal breaker.
Crap docs aren't always the fault of the guys implementing though, sometimes there are time constraints that prevent proper docs being written. Quite frequently though its outsourced development agencies that refuse to write it because its "out of scope" and a "billable extra". Which I think is an egregious stance...doxs Should be part and parcel of the project. Mandatory.