Well no, I don't think he's that good. Just before 09/10 started, many of us (myself included) felt confident of a title challenge. The fact that our main backup for the then world class Fernando Torres was a player who is now struggling to score goals in a lower division really tells us a lot.
Also, players' performances elsewhere are not necessarily an indication of their form or quality when they play for us. When Morientes joined us, he didn't live up to most peoples' expectations, but I doubt many Madrid fans saw that and retrospectively decided he wasn't as good a player for them. It depends on the quality of the other team and the system they play in, among a number of factors. The 'fact that our main backup... was a player who is now struggling to score goals in a lower division' could just as easily tell us that he was a promising player whose development was negatively impacted by unfortunate circumstances.
I'm not sure there's really that much of a debate to be had re Ngog, to be honest.
(a) He was a promising striker brought in for a fairly low fee.
(b) I don't think anyone expected him to have the potential to replace Torres, but there were expectations that he could go on to be a decent Premier League striker and a useful option for us.
(c) He was forced to play far too many games, and take on far too much responsibility due to Torres' injuries, and much like the aforementioned Insua, this proved detrimental to his development.
(d) That said, he still displayed flashes of the quality that many hoped to see more consistently in him.
(e) Unfortunately this consistency never quite materialised during his time at Liverpool.
(f) We took a look at his development and his abilities, decided he wasn't good enough, and sought to move him on so that (i) we could make some money and (ii) he could play more often.
Promising player arrives, plays well, doesn't quite meet expectations, moves on. See also, in varying degrees, Mark Gonzalez, Insua, Babel, Plessis, El Zhar, Nemeth.... and perhaps Danny Wilson.