Here is what some one would say from the old school about transformers for inverters. Transformers themselves make great inverters, they are very efficient, very reliable and produce very clean sign wave. In a since Tesla, westing house, Tomas Edison have been working with clean 60 cycle transformers for years. The problem is to get a clean solid state driver to work the transformer. High current liner transistors can to the job very well,but they are very expensive and are limited to about 40 amps. This is not enough current at 12 volt to a lot. So a type of SCR switching transistors is used. They can be good for almost 1000 amps. But they just switch the transformer on and off 120 time a second to sort of give you a square 60 cycle wave. So I say it not necessary a bad transformers, but it is bad solid state stuff that is use against tranformer.