I can only compare the Bronica SQ to the Pentax 645. First, when it comes to loading film into either of the inserts, I find the Pentax far less cumbersome (going to use that term a few times here).
The Pentax has a nice hinged roll holder. Easily flipped up and down. The Bronicas have to be pushed out. Depending on the style and age of your insert, may be loose or stiff.
As for getting the film started, with Pentax, plenty to grab onto, and doesn't feel like you're going to break anything. one thumb inserts the tongue, other thumb rotates the gear. Insert pushed in, lock turns.
With Bronica, it's like a Skeleton. Always feels fragile, and very cumbersome. And when it comes time to put the insert in, you have to pay attention to what side, and make sure the gear goes into the slot for it. Then without it falling out, you have to close the back.
However, that all has to do with loading the film INTO the insert. Easy with Pentax, pain with Bronica. I hate having to load film on location with my Bronica.
I do like the Bronica backs as they're entirely self-contained. The Pentax inserts need a box to ensure their safety.
Putting the Bronica backs on with the prism viewfinders is often an annoyance. You have to insert the two lip catches at the right angle, and have to line them up with the holes. All while you can't see what you're doing because the prism is blocking view. I've scratched mine up to hell.
The Pentax inserts just push in, and you turn the lock.
Prefer my Bronica, but it's definately a pain when it comes to loading film. That I cannot deny. The Pentax though is my workhorse. I own three of them, and two sets of glass.