I've never been on the Orient Express*, nor on the Thello (which unfortunately seems to have had some negative reviews, although it still seems to be better than the Artesia trains it replaced...) However, I am not sure you can claim that something is "very overrated" without specifying by whom, or in comparison to what.

I regularly travel by overnight train in Europe (normally either in 4-berth couchette or 2-berth sleeper). Admittedly, that means I know what to expect, so I am unlikely to be either very disappointed, or extremely impressed. And perhaps I am also fortunate in that I can sleep well on a moving train, and haven't experienced a serious problem like a broken ac, as davewang202 did.

However, overall I find overnight trains in Europe to be perfectly adequate, and indeed they are my preferred option for medium-distance travel within Europe (as they are generally time and cost effective, compared both to daytime train travel, and to flying).

* I assume you mean the luxury Venice-Simplon Orient Express, not the discontinued EN469...