Ah, we have just ripped one out. They can be very crap but more in terms of design than quality IMO.
The frame is always attached to the front of the cupboard. The door is fixed to the frame with butt hinges (better) or fixed back to the carcase with the kind of elbow hinges you have on ordinary cupboards - cheaper but much bulkier.
If a cupboard is only 30cm or 40cm wide, you put a 3cm or 4cm frame all round it, and if you have those elbow hinges the door doesn't open clear of the frame, you can end up losing over 10cm of your width and I couldn't fit pans or boxes in many of my cupboards.
If the cupboard has drawers they are much narrower to fit inside the frame - more wasted space. If it is a double door cupboard, a cheap one will have 2 frames next to each other so you have a big divider down the middle even when both doors are open, rather than a better one where there is one frame with 2 doors fitted in.
Corner cupboards can be horrendously inaccessible, especially if you have a drawer at the top, and the pull-out fittings may not work.
So the moral of the story is - make sure you don't have narrow cupboards, don't have drawers at the top, have butt hinges, look for decent double cupboards, basically get it designed to work with in frame units and not ordinary units with the doors swapped over. You could do that with even a cheap range and get a decent kitchen.