The iPhone Duo gets very hot under sustained load, such as when the camera is used for extended periods or when gaming. The volume buttons on the top are reportedly difficult to reach, particularly during a phone call, while the Touch ID fingerprint sensor in the lock button does not always work reliably. At the moment, unoptimized third-party apps still cause problems, for example when they are displayed as a small window with black bars or rotated by 90 degrees, though many of these issues will likely be fixed through updates.
Joanna Stern used a small machine to fold and unfold the iPhone Duo more than 32,000 times, which caused no problems whatsoever – although, according to a measuring device, the hinge offers slightly more resistance than before this test. All in all, while most reviews describe the iPhone Duo as a solid foldable with excellent build quality, Apple's first-generation foldable phone is not entirely free of problems, as the reviews from 9to5Mac, The Verge, PetaPixel and Marques Brownlee are showing.