In the diagram, you can see the difference between the pure sine and the modified sine. You will note that the “blocky” modified sine spends longer at the top and bottom of the wave as opposed to the pure sine that just touches the top and bottom. The extra time at the top and bottom of the modified wave has two main impacts.
One is heat, a longer time at the top and bottom cause heat in sensitive electronics and can damage them irreversibly.
Two, anything with a microprocessor counts the sine waves as part of its processing, and the blocky waves can confuse the processor and may cause it to malfunction.
Some will have found that a device will charge OK, but if turned on while charging, will not operate in the normal manner. That will be because it is modified sine.
The grid electricity in homes is pure sine, and pure sine is the only way to be sure your electronic equipment will operate correctly in your MH.