The one that I had was on 5 minutes out of 7 hours to make up for heat loss. This comes out to a 54 W loss through the insulation so it was 100x(4500 W-54 W)/(4500 W) = 98.8% efficient.
They're all like that.
I'd ask how much parts cost, those parts that are most likely to fail.
You could also search for class action lawsuits against the manufacturers to weed out some candidates.
There are probably no reasons external to the WH that would cause a WH failure, but do you know why the old one failed?