I crunched some numbers a couple months ago and came to the conclusion that piling on is the best skill for taking people out. It has downsides, but if all you care about is getting a numbers advantage piling seemed like the way to go. I've now played a fair number of games with the piling on skill and am probably in a better spot to look at the downsides.
The biggest downside for me is that typically my guy with piling on is going to be the one I most want to hit with. He probably has mighty blow and/or tackle and few other players on my team are likely to have those skills. This means I really want to be blocking or blitzing with him every turn in order to get the numbers advantage. But if he piles on he removed himself from blocking on the next turn, and he makes it harder to blitz with since he has to spend 3 of his movement standing up. This restricts how far he can walk, which limits his potential targets on the next turn. Also, if piling on didn't pierce armour, the guy he hit last turn is going to be standing beside him this turn. This means blitzing is going to require a dodge, or having someone else clear away the other dude.
Getting fouled has actually not been a terribly big deal. I can do a reasonable job of putting other guys in the area to take away potential fouling assists, and it turns out most people just don't have dirty player on their team. This is a big mistake I think, but I'm happy to take advantage of it. On teams that do have dirty player they tend to become target #1 for the piling on dude. Typically dirty players won't have other skills or defensive stats so they're easy to take out! And if I end up having to stand beside an enemy in order to get the blitz in I actually find I'm happier being on the ground instead of standing up where I can be trivially hit back.
One of my piling on dudes did spectacularly well. He scored up 7 casualties in one game, leveled again, rolled doubles, and got jump up. That guy was incredible. He could piling on every turn, and then if the enemy stood up he could just do it again without eating up my blitz action. Of course he was also a lightning rod for the enemy teams and died not too long afterwards. Oh well!
For the most part I feel like the downsides of piling on are actually downsides. When it works out and guys fall off the pitch it's such a huge advantage, but if you piling on a couple times without removing anyone (which is an entirely reasonable outcome) I find positions tend to fall apart. I want to have more guys standing every turn if I'm going to win positioning battles, and I want to be able to blitz down key targets. It's definitely a high risk, high reward skill. I would never take it over mighty blow. Even though mighty blow has slightly worse numbers the fact that you stay standing and can therefore keep blocking them if they stand up makes it better I think. My dwarves still don't have anyone with piling on, but have 4 guys with mighty blow, and have no problem winning games by just punching as many times as possible every turn.
But once you have mighty blow, piling on is still really good. It's risky, and I now feel like just using it every opportunity is probably wrong. If you knock a generic dude down and don't break his armour then you may well be better off staying standing in order to hit him again next turn instead of piling on and hoping. But if the target is important it may totally be worth going all-in and trying to hurt him right now. My human team often wins by getting a man advantage so having piling on and trying to get lucky seems like a reasonable course of action. On the other hand my Skaven team probably shouldn't be winning via attrition (though it totally is) and therefore may not have been a good place for piling on. On the other hand they can get claw too, so I can get the best killer in the game! And while it may not be game winning to piling on a few zombies or something I can see winning games by selectively removing my opponent's best football players. Piling on into ghouls or skinks would be pretty hot!