UPS units can be frequency sensitive but when I tested mine I hooked a meter to it, Voltage and Frequency were both good on the true RMS meter. but I am guessing waveform.. The Genrac 1000 has a rating of 3% THD or less on the waveform, but ... there are components that help to shape that wave, and some of them can "Age" if allowed to set too long... They tend to dry out or something (We talk about them drying thought that may not be what happens). once you work the genny a while they shape up and work properly.
This is a real problem for vintage radio lovers cause if i were to plug in say an Eico 720 (My first Novice Radio Transmitter) I would want to use a Variac to bring the voltage up SLOWLY to insure I did not blow those very same parts in the transmitter. (yes, since it produces a waveform)
http://twit.tv/hn and I think it is the last program (Dec 9 2014) one of the hosts fires up HIS Eico 720 Nice rig, His was set up for AM though, mine only morse code