It must be clear by my many posts in this thread that I do so love IEMs.
The nature of an in-ear bypasses the outer ear, which greatly changes how we perceive sound. I always find over-ears more natural sounding simply by this virtue. There are plenty of things they can do wrong (same for IEMs) that make them sound less natural/pleasing, but the big drivers emitting sound from outside your heard gives them a good lead over in-ears.
It would be disingenuous of me to say my $1000+ IEMs sound better than my $200 HD6XX. There are things I like more about the U12, but overall it's not a fair fight. IEMs are just working from a handicap, due to their size and in-ear placement. The gap grows worse when you compare against something like the Audeze LCD-2. Any IEM is shamed against The Sound Wall.
I will say, the ADEL tech helps the U12 come much closer to full-size cans than anything else I've heard. And of course, 12 great drivers tuned to perfection makes a difference.