Kings Dominion, Cedar Point and Valleyfair come to mind as being on time.
I guess you're right (though two of those haven't opened yet). Cedar Fair usually sets the standard when it comes to being on time with their new stuff, but Dorney, Worlds of Fun, CGA, Carowinds, and (kinda) Knott's were late this year.
If you thought Cedar Fair was late, you wouldn't want to know what Six Flags' new rides look like right now (Hint: None of them are open).
Anything before Memorial Day is nowhere near being late. Patriot opened a week after the park opened, and Knott's never closes so there is no "late"