Excellent example of this was when Jolla's own stskeeps (the guy responsible for libhybris, which UT also employs) took issue with incomplete source (actually just mislabelled parts of source) being released for the Aquaris E4.5 in a blog post and post to the dev list. The developers responded and said they'd look into it immediately. They found the headers were incorrect (rather than Mediatek / bQ removing or obfuscating bits of the source) and released a correctly labelled version of the source within days. Granted you can't see the development process for OEMs' work, though you frequently can test their internal builds which are often pushed to the main repos. They're as transparent as it gets.