Are you pushing them too hard? You need to let the rpm stay up. If you bog them they will burn up due to high electrical draw.
Use a grinding wheel (approx. 1/4 thick) for grinding on the flat side of the wheel, and a thin (approx. 0.040 thk) cutting wheel for straight plunging cuts. Don't force it, let the wheel spin and cut.
That said, I have no experience with these cheaper tools. When I buy a tool I expect it to last, so spend the appropriate amount.
My Black and Decker (Sears) has served me well for years of major projects interspersed between long idle times; and the Milwaukee's and Ridgids we have at work seem to last.
My friend Karl has had good luck with Makitas in constant use.