Steel string guitars and nylon string guitars have different string tensions and neck shapes. Steel strings are typically much tighter than nylon strings, which means that a capo designed for steel strings may be too tight for a nylon guitar, potentially causing the neck to bow or warp. Conversely, a capo designed for nylon strings may not be tight enough to hold the strings down properly on a steel string guitar, which could result in buzzing or intonation problems.
Additionally, the type of material used in a capo can also affect the instrument, especially when switching capo types (from nylon to steel or vice versa). Capos made for steel strings are usually constructed of stronger and harder material to handle the tension, while nylon guitar capo tend to be made of lightweight material to reduce any unwanted tension on the strings.