This is entirely unscientific, but may be interesting to some players or potential players.
Numerous Leagues and players have commented over the years about the "worst" team in Blood Bowl. Obviously, everyone has his own opinion. A team that one player can consistenly win with may be an utter impossibility for others; likewise, a team that is very popular with players may, in fact, score very little. A perfect example of this paradox would be the Halfling team - it has appeared in virtually every official version of Blood Bowl, and while many coaches over the years have claimed that the Halflings are the worst team in the game, they remain, conversely, one of the best selling Blood Bowl teams, and a huge number of Blood Bowl players at least own the Halflings, even if they decide very early not to use them in any "serious" game.
There is also the matter of the variation between playability in certain teams between various versions of the game. That is to say, even if a team's stats remain relatively similar, a team that does particularly well, in say, 3rd edition Blood Bowl, might be an absolute disaster in the v6 rules. Conversely, some teams that have been historically regarded as truely awful have, in fact, disappeared from the game, or have been completely re-done, so it's difficult to gauge how they compete for worst of the worst.
All that said, after a lot of research, questions, and simply lurking in Internet forums, the following four teams seem to be top candidates for the worst team in Blood Bowl, in no particular order. Note that not all of them are rated as "Advanced", and that others, in fact, have a reputation for winning a lot of games under the right coaches. However, they do give a fairly good picture of player opinion.
As we gather more scientific statistics here at the BBWiki, we'll be sure to post them.