It's not just the union as charter schools are (in general) just as mixed of a bag as public schools. There was a Stanford study back in 2009 that spanned 16 states and showed around 20 percent of charters outperformed and 35 percent of charters underperformed their peer public schools. Actually this reminds me they were supposed to have another round of outcomes last year or so. I'll have to go and see again how things have changed. (Offhand, it wouldn't surprise me if it's relatively close overall).
