I have been on Sunwing many times and sold it many times. Of course this is from Canada (and always to Mexico) and I haven't encountered any flight delays, but I have had clients who have, but no more or less than any other charter airline in Canada. They also do have less room on their planes as do all other charters in Canada, but flying to Mexico on cheap airlines from here is what we've gotten used to I guess. You can upgrade to elite class on their planes and get more leg room if you like. I believe it is about $160 per direction per person, but this is all taking from what they have done from Canada. This is very new them flying out of the US, so I don't know specifically about that. I'm also not 100% sure it is Sunwing flying for Vacation Express or Vacation Express leasing their planes and having their own services. I would check into that.
Because when they fly from here you still get a free meal, their is pay liquor service, they don't have seatback TV's on any route I've been on.
Anyway, I"ve flown them, would fly them again and don't think they are better or worse than any other charter.
I;m sure you'll be fine:)