The issue with most UPS system is the windows are set for utility power. Those windows can be opened on most UPS systems to except the voltage an Hertz differences, as well as the fluctuations. The UPS end product will be the same. Should the issue be transients do to alternator design, a small unity power factor load can be added or in some cases a Cap will correct the gen set wave form enough the UPS will except same.
Once you know how a device, system, ect operates, finding the root cause in the event chain is easy. Until then, you will always be chasing/repairing the effects. Learning is a life time event, if you want to outsmart, whatever item you want to repair.