I have fed chicks to my balls on occasion when no rats available, just to get my snakes by.
As for long term effect, i got a 4 year old female 2 months ago that was regularly fed chicks since it was big enough to swallow them, it was 1100 grams when i got it, after 2 months of eating rats with me it's now at 1350 grams empty. I can't say it's a definitive proof of chicks being a poor choice for balls, but it's what my scale is telling me. A fellow herper who wanted to save on feeding cost tried chicks for a few weeks, returned to rats after seeing unsatisfactory weight gain on his snakes. That last part is 2nd hand info, but i have no reason to doubt it.
I also observed that snakes have more difficulty swallowing a chick than a rat of the same weight, because chicks have larger diameter as opposed to rodents being elongated.