Where rubidium standards are designed to be used for frequency standards, it is hard to see what the objection might be. If it were set up for an odd output frequency, required a digital rather than analog tuning signal, or output a square rather than a sine wave, that might be a problem depending on the application. Most of the standard LPRO-101 class units require a EFC voltage and output 10Mhz sine wave so they are a good choice if you're buying a cheap used rubidium standard. A buffer/DA is always a good idea as well.