So, I think these points have been already made, but let's just tally them up for a second...
1. Impact hits only get bonus from full ranks, so a 1x3 formation ("dart") only does Str4 impact hit(s).
2. Champs do
not have to accept challenges, it says in the BRB although I can't remember the page...
Now, the other point that hasn't been brought up yet...
3*. Three Bulls with Ironfists and a champ = 106 points. Three independent Sabretusks = 63 points altogether.
As far as points are concerned, Sabres are the best bang-for-buck value. Now, if you're having trouble keeping them on the board via arrows or terror-bombs, keep in mind that the Bull-chaff only needs to lose 3 wounds for a panic test; granted, their LD7 with possible bubble influence is better than LD4 no-help, but if they panic you're losing more than 5 times the point-values if the Bulls run over a Sabretusk.
Granted, for anti-chaff/counter-chaff tactics, I think running this is adequate, maybe one or two units. They can fight smaller units relatively alright, and if you care to give them a magical buff during a key fight they have a good chance of actually winning a combat or two. But, for just plan regular chaff purposes, they're a lot more expensive for something that is only slightly better than what the Sabretusks can do.
I actually ran one unit of three Bulls (w/AHW and champ, for a whopping 13 attacks!) in my most recent tournament, and I found them to be
alright, but not the most amazing... But, the three opponents I went up against didn't have much in the way of chaff for the Bulls to go after

33 Gnobs! Over 3 Packs of Trappers! Woot!
Gibblet, Nibblet, Crunchybit (from Slaughtermaster achievement)
New book WLD: 20/10/2 - Yay tourney wins and random game win!