If you are going to use it in a field, or mobile it's just way more convenient to use a handheld oscilloscope. One that you can charge from car battery cigarette lighter or USB is even more flexible.
The only thing stopping you from a bench oscilloscope is noise created from the UPS or inverter, safety implications if you are checking dangerous voltages, wrecking the supply due to lack of earth or dirty power, possible environmental issues due to the oscilloscope having unprotected LCD screen and open vents for moisture plus dust etc to get in, not to mention being awkward to use out in the open.
A bench oscilloscope with a battery option if the environment or convenience isn't such an issue.