First I tip my hat to @
Axel_Erfurt comment as it is basically inline with my thoughts on it
But to be specific @
Mocap it is like this -- one of the major elements to how pyqt was designed to be used is Layouts -- now while you can use Layouts while developing a GUI in Designer when you are done and you get that mess of code the Designer barfs out you will notice that their is next to no Layouts (and often not a single one) within that code -- instead it uses a sort of static coordinate system which has issues. So in order of issues they are:
1) The Designer does not use the essential Layout system of pyqt in the code it produces
2) The code the Designer produces is extremely ugly and thus hard to debug and maintain
3) The code the Designer produces is filled with inessential items making it unnecessarily complex
4) The code the Designer produces does not use Class encapsulation as it should in conjunction with pyqt
5) Creating bad code with the Designer takes as much time or more time than creating good code without it
Those are my issues with that tool - yes some folks use it but I would argue that ought never be used because that the reason I hear for its use is often simply a case of programmer laziness and inefficiency. Something that could easily be adjusted if the programmer were to be a bit more thoughtful about what they were doing.