I have a 3120 with a 24", a 36", and 50" bar. All in .404 guage, full comp chain. It's my favorite saw, but you don't dare run it without the earplugs handy. A complete thrill to use when you have it with the 24" bar chopping up a smaller tree on the ground. Not really any fun at all to use when you are 20' up on a huge oak dropping 4' diameter log chunks.
Yes, the big bar has a greater tendency to curve in a cut, and I think it is a matter of ratios. The height of the 50" bar is not much different than for any other bar, but it is much longer. So it's height:length is a much smaller number than for almost any other saw. Imagine how big a curve you could put in a cut if your 24" bar was only 1/2 as tall as it is now?
Bowing on the cut has never been a problem for me, but it does tend to throw the chain very quickly if you are a bit loose on the chain and then gun the throttle or drop it into the wood while you are horizontal. Start the cut way out on the tip of the bar, then walk it forward on the trunk until you are up to the dogs, then dig in and cut a big swath! That way the weight of the bar is always suspended by the tree, and the chain doesn't get any chances to jump out of the groove.
My biggest problem with the big bar is that it seems to suffer from more wear on the chain and the bar than other situations (smaller saw or shorter chain). I have always presumed that is because it has a LOT more horsepower going into a bigger load than other saws.
Strangely: the big bar often gets wedged in the cut by sawdust. You need to be careful to keep reaming the horizontal cuts to keep the chips clear, otherwise you often develop a tight pack of chips between the bar and the wood right at the tip. Once trapped, it is dam difficult to get unstuck. I have always supposed that it is because of the large volume of chips that the saw makes in the cut.
I've had it since 1996, and it has always run great, although the bar studs get too much stress on them when you use the big bar. The holes in the crankcase have wallowed out now so that the bar studs fall into the oil tank if you are not completely carefull to prevent it. I would mushroom/oval/knurl the studs, but Husqy makes them too hard. Besides, getting too rough with the studs will just continue to enlarge the holes in the crankcase.