Film flatness was a problem for 126, but my understanding is that Kodak addressed the problem in 110 cartridges by holding the film under tension so that it produced a consistent curve. The lenses were then formulated to that curve/buckle in the film, so the film was theoretically "flat."
The problem, even with 126, was never that the cassette could not be inserted perfectly flat. The problem with the 126 format was that the film channel in the cassette was insufficient to hold the film in the exposure gate was not held flat enough due to its thickness, height and width, and the influence of the backing paper. The smaller, thicker film used in 110 cartridges eliminated this problem: the film channel was more precisely dimensioned and the film stiffer so that it didn't buckle in the gate.
The other problem with many of these cameras (both 126 and 110) was that the quality of the lenses used in most models was insufficient for high magnification printing. I never tried any of the higher-end 126 cameras as the 126 cassette wasn't really up to the task, but this was not true of the Minolta 110 Zoom SLR that I had, from which I could easily make good 8x10 prints, and occasionally an acceptable 11x14.
None of the 110 cameras have the brilliant film handling or superb quality lens of a Minox IIIS, B, or C, however. Well exposed and processed Minox 8x11 negatives on fine-grained film can easily make 16x20 inch sized enlargements due to the way the film is managed in the exposure gate and the superb quality of the lens, despite the diminutive size of the negative.
😉
G
Note: A current iPhone 15 Pro camera is in a very different class of performance. But trying to equate film and digital "quality" is an absurd thing. The capture mediums are simply too different, IMO. I enjoy both mediums, and all the various formats in both mediums, for what they each can do, how they each interpret what I see differently.
😀