another con - they are getting harder to find. distributors starting to not carry them, so the price is only going up.
you can sometimes get 5gallon cornies for $20-$25, vs. $50 for enough mini-kegs to store the same volume of beer.
and co2 cartridges become more expensive than a 'real' CO2 tap run off a 5lb cylinder (or even 20oz paintball tanks).
I also found that mini-kegs rust the instant you scuff them, inside or out. SS kegs don't do that.