Qt 5.6 and later switched from LGPL v2.1 to v3. The main difference is the addition of the anti-tivoization clause that the regular GPLv3 has, wherein you cannot sell hardware that integrated Qt without providing a way for the user to replace your copy of the libraries with their own. Although at that point most people just buy a commercial license, and I think the majority of people that complain about this don't have a legitimate use-case that really affects them as much as they lead on.