You get kicked out if you roll doubles on the 2d6 armour roll, or if you roll doubles on the 2d6 injury roll if you pierced armour. When you foul you do get +1 to the armour roll for every adjacent assist. This means you're looking at needing to roll anywhere from a 10 to a -1 on 2d6 in order to get to that second 2d6 roll. I made a nice little table...
Each column indicates the amount you need to roll on the 2d6 after taking assists and enemy armour into account.
So, for example, if you run up and foul someone who has 9 armour, and you have no assists at all, you're looking at a 19% chance that you get ejected yourself vs only a 7% chance of at least knocking the other guy out. The odds only swing in the fouler's favour at the 'roll a 6' mark. But sometimes just stunning an opponent is good enough (say there's a guy on the ground inside your ball carrier's cage, for example) and you have pretty good odds of doing that with just a small number of assists.
There are a couple of skills that can be used with fouling. Sneaky git means you don't get ejected unless you actually broke their armour regardless of the rolls. The table then becomes...
Someone with sneaky git looks to always have a good foul action regardless of assists or armour. There's still a chance they get kicked out after only stunning the enemy but that looks to be more than offset by the times you KO+ the enemy without getting ejected yourself. But at the left end of the table, when you get lots of assists, it doesn't really do much at all. Sneaky git only decreases the odds of the bad outcome, it doesn't increase the odds of the good outcome. The only reason I'd consider taking sneaky git is if you have really terrible players without general skill access. Like, say, halflings. You want to foul every single turn with the halflings and unless you can get a ton of assists it's probably a good thing to have a sneaky git doing the fouling for you. But on any sort of real player I can't imagine taking it.
The other fouling skill is dirty player. This gives you +1 to your armour roll, and then if you broke armour and didn't need the +1 you get +1 to the injury roll. This changes the table as follows...
Well... Dirty player sure seems good. Now the break even point is at needing to roll a 9, which is pretty much an unassisted foul on anyone with 8 or less armour. Get a few assists so that you only need to roll a 5 and you're looking at a KO+ more than half the time, and a casualty a quarter of the time. You'll get taken out yourself 30% of the time, which is a lot, but if you have a cheap lineman with only the dirty player skill this seems entirely reasonable. Trade in your 60k or 70k player for someone who may be worth 150k or more. That is a very solid investment. Heck, even my dwarf team could make very favourable trades with a 90k dirty player.
Ok, I'm convinced now. Deathroller is definitely getting cut for 2 regular dudes who are going to put the boots to the best players on the enemy team if it makes sense. Come here, Olaf...