I'll second the thought that rollers get weighed more than backpacks. I've experienced that myself as far back as 2009 in New Zealand. I had a 31 pound backpack on my back, while my wife had her roller bag. They weighed hers and forced us back to check it. Looked in my direction and I just acted like it was OK, didn't need weighing.
I use a Thinktank Speed Racer waist bag AND either a Thinktank backpack (above water cameras) or a Thinktank roller (D850 inside Nauticam housing with 230mm dome port taking up the bulk of the space. Any of this luggage will fit under a seat in a pinch.
I put the housing in vertically, and I do have to remove the two ball mounts that stick up highest. (The ones I use with a rope handle for carrying). The real problem is the dome port. Once that goes in, everything else has to fit around it.
I can get the housing, camera, focus light, dual strobes, 230mm dome port (no extension), vacuum pump, two 5 inch arm segments, and all the batteries in the roller bag. I put three lenses in my waist bag, plus sometimes a RX100 II inside a nauticam housing as backup camera. That still leaves me the macro port, the 90mm dome port extension and all the arm clamps to go into a suitcase. I put 7 arm clamps into the 90mm extension to save space.
If not for the dome port, all would fit into the roller bag. If not for the rollers, all 'might' fit in - the wheels and handle take up a lot of space that is present on the backpack. But I've never tried to put all the dive camera gear into the backpack because I don't want to lift that on my back anymore.
That said, it would be an excellent idea to have a roller than had a backpack option so you could carry it in a pinch, if only to leave hands free to pull two other suitcases.