I briefly started investigating the changes for Qt 5.7 before I started having flashbacks of trying to figure out all this LGPL stuff again. Does not seem to be any less confusing...
Before, you had to maintain a Qt source repository that matched distribution. People could build and link in Qt as they pleased, either vanilla from that repo or modified as they liked.
With 5.7 and the new LGPL, it seemed that you also have to provide the entire toolchain needed to build the source and link it into the final application. Which means you also need to provide the kernel headers and etc that match up.
I could be totally wrong. But I'm guessing 5.7 required too much other stuff be made public and the customer wasn't pleased with that. Can't say I disagree.
Of course, part of the reason why The Qt Company likes LGPL3 is that other companies are going to look at the requirements to use and distribute open toolchains for their supported platforms and just buy a commercial license instead.
Qt seems to be almost caught up with getting qt5 in the meta directories and having everything in place.