From what I've read, they're number 4 in Italy.
They're powerful, although not quite as powerful as Ndrangheta and the others, the Sacra Corona Unita is a relatively 'young' gang. They were founded in Apulia by a Neapolitan who wanted to expand his operations, but since then they're composed of born and raised Apulians. They are active in Italy, but they've also spread throughout Europe and North America.
They have some very strong allies : apart from the other Italian criminal organizations, they're also allied with the 'Russian', Albanian, Colombian, Triad and Yakuza crime organizations as well as working with Moroccan immigrant gangs in Italy.