I would think they're safe, since everything's "clamped" together. How well they work is another question. If you chase and face the head tube properly, and these things press in, then you'd be pressing in the cups on something already pressed in. Off-concentricity from tolerance stack-up might make for a less than ideal headset adjustment, but then, a lot of production bikes were built with poorly faced and chased head tubes, and they're rideable.