The 3d free-rotation for this view camera is a traditionally awkward way (in my opinion) for rotating. I've used different 3d modeling applications, few that do it this way, but I never liked it. I think the early Photoshop 3d viewport used this, but now does something else. Lightwave modeler always had one of the best free-rotation cameras.
Off-hand, I don't remember what that rotation equation is to be most effective.
Yes, the problem with this method is that the final orientation depends on the path you cursor takes, i.e. dragging up 100px and left 100px is different than dragging left 100px and up 100px...
Of course, that's just the way 3D rotations work: SO(3) [1] is non-Abelian. However, doing this with an input device that has one too few degrees of freedom makes it very awkward to use.
When you have a well-defined "ground" plane in your scene, then turntable-style rotation is the most intuitive, where horizontal dragging rotates around the ground plane's normal, while vertical dragging changes the azimuth angle. This makes the final orientation independent of the cursor's path, on the other hand, orientations where the ground plane would be skewed are unreachable.
Some people swear on Ken Shoemake's ARCBALL [2] technique, but I personally found it less intuitive than the turntable method (although I might have implemented it incorrectly ¯\_(ツ)_/¯).
For viewing objects I find a turntable style view best in most circumstances. For landscape or architecture (where you're in the scene), mouselook with WASD + vertical up/down.
Neither of these control styles are 6dof; they don't provide a roll control, only elevation, azimuth and position. Roll is rarely useful IMHO and including it in the normal control scheme makes things hard to use. Put roll behind an explicit modifier key, and have a reset-roll action somewhere.
Off-hand, I don't remember what that rotation equation is to be most effective.