I own a Conn cornet that Michael Del Quadro modified with a beadless copper bell, so I have some "owner's experience" with beadless copper bells from different sources. The bells on both the NYTC trumpet and the Del Quadro cornet were plenty hard, and neither seemed more fragile than a brass bell with a bead. I also own a Calicchio flugelhorn with a solid copper bell which was formed traditionally from copper sheet, and that bell is thick, heavy, and soft. I have to assume the thickness is in some degree to compensate for the softness, but the flugel bell has picked up dings much more easily then either electroformed bell.
While I have no reservations about the strength of these bells, I do think taking full advantage of them involves experimentation and prototyping. The Magnum trumpet had no doubt gone through many iterations and design refinements to result in a horn that I really could have played to this day -- nobody ever said to me, "Dude, that trumpet just doesn't work for lead playing. Get something else or find a new band!" I simply found that *I* wanted something different, and a different trumpet that happened to have a bell with a bead delivered an indefinable difference I preferred. The Del Quadro-modified cornet is a unique beast that reveals its one-off roots: It plays great, I love playing it, but it only works in certain settings. It projects more like a flugelhorn -- the sound is diffuse and seems to project as much back as forward -- though it doesn't sound like a flugel. I really enjoy playing it in small venues, playing small-group jazz.