Keys work, kind of. If I only have one or two to do at home, they work ok. If I have dozens to do at work, it's possible, but slow and painful.

They're difficult for me to grip comfortably. About half the time, the tape gets caught in the teeth of the keys, then starts to rip the tape, then the top layer of cardboard off. I usually have to stop and pick tape, gunk, and debris out of the teeth every 2-3 boxes. Once, some tape stuck on the side of my key and I didn't notice until I put the key in the lock and the tape got bound inside the mechanism.

A knife isn't ideal either. I sharpen my own knives so I hate using them for paper and cardboard. It's very tough on the edge and usually not worth it. So I split the difference by using the back of the blade on boxes. If I know ahead of time, I'll bring a utility knife with a disposable blade to use.

Probably the most common thing I do with my edc knife is slice up fruit for myself or my dog (yes, I make sure it's clean first). I haven't found a set of keys that will do that yet.