Why is it bad to have an inductive load (context: dc-dc converter)?
Adding to Dave Tweeds good answer, it is the act of switching OFF a reactive load that can cause major problems. Most common would be a large inductor or solenoid or motor with large inductive values.

They store energy as a magnetic field and when the power is removed the field collapses, returning the same current back into the wires. If there is no path to discharge this energy it will normally arc across the switch or relay contacts, causing the need to replace them every few months or years. The crux of the problem is that the voltage (back-emf) will rise until it finds a discharge path.

If the source of power is an electronic device such as a UPS then damage can occur over time, as they have just simple 20 mm MOV protection against voltage spikes. Any electronic device or AC outlets that drive a heavy reactive load should have strong surge protection.

NOTE: A UPS in general is NOT designed to drive heavy reactive loads, even though it may have twice the current rating of the load. A UPS is designed to drive wall-wart transformers or switch-mode supplies with little to no reactive loading.