Small lightweight UPS, without transformer?

Rather called Power Inverters which are quite small and light weight as they dont have a heavy weight transformer. And upto 20A battery charger. Available from 1000 to 8000 watts (modified sine wave). Also quite cheap (chinese).

Has anyone used these power inverters instead of trendy UPS?

How do they perform?

Are these a better choice instead of buying a desi made square wave UPS?