My guess is there is a patent on the Engels, which they aren't licensing.

Most of the literature I see on the danfoss compressors is that it is now controlled by an electronic board. One bloke believes that prolonged low voltage kills this board.

The waeco UK Site says it happens to all compressors.

"
Also, if the supply voltage from the battery becomes too low, electronics and motors tend to overheat. This occurs with all electronics and all motors no matter who the manufacturer is. And that is why we do not allow this to happen to WAECO products.
"

From http://www.waeco.co.uk/page.aspx?page=faq&s=4

Also on the same page, some models (or maybe all models?) have a 'my beer isn't cold enough switch' (emergency) which bypasses this elec-trickery controlling/protecting the compressor. You are on you own then. I was unaware of the electronic protection in my earliest post.

I'm unaware of the reason for warranty time differences. You know how we only have so many heartbeats in a lifetime...