Programs with complex Reaction GUIs don't exist :-)
but if you create one for the shake of it you will notice that these are not faster than MUI and almost block intuition while they are being drawn.
From a coder viewpoint using Reaction means reinventing the wheel over and over: the classes are too simple, each programmer has to implement the same things if you want to choose screenmodes, fonts, backdrops... and there few available classes. Drag'n'drop is crude and primitive with Reaction.
After having written various Reaction GUIs I think that Reaction is some kind of enhanced Gadtools with automatic gadget resizing.
I heard more arguments against Reaction like it blocking input.device (perhaps that's the Reason it almost freezes your computer when it draws a big GUI)
MUI is more elegant:*you don't have to create ultralong event loops like you do in Reaction.
MUI*is more portable:*it runs on a plain 68000 Amiga, comes as standard with OS4.x, AROS is MUI compatible and it's bundled with MorphOS too. In contrast Reaction doesn't.
PS:*IBrowse1.22 was better than AWeb in the old times :-P
