The inflexibility I find is because the brood frames are not all the same size. If you want to move them around to open up the brood nest, you are limited to manipulations in the single boxes only - you can’t move frames up from brood to half brood, or vice versa.
Also if you have brood mainly in the half brood and need a frame of eggs and larvae for another hive, but for the brood box, it won’t fit - you have a gap beneath. That sort of issue.
If you read Michael Bush “The Practical Beekeeper” he goes into the benefits of one frame size throughout the hive - brood and supers (!) for total flexibility. His stuff is available free on the net. It’s very sensible I think.
Also, I’ve found on brood and a half, I have had to inspect both boxes frame by frame for swarm control, as the bees quickly outgrow the half brood if on top. On double brood there is often little need to thoroughly check the bottom box as queen cells are usually located in the upper box. Commercial keepers on double literally tilt the upper box and inspect the bottoms of the frames and call it a Doris.
I hope that helps and I hope I’ve explained it ok?
A lot of folk operate brood and a half as the brood area is suited to most colonies, so my comments are personal, not critical of other folks methods