i've noticed absolutely no flex, have had several pike that were in the 13 to 15lb range and numerous bass both large and smallies of decent size many in cover. i would bass fish this reel anywhere with out a worry. the only thing i am not a fan of is the newer shimano handles so all my reels get hawgtechs with septons and a good degreasing and proper tune right out of the box. besides that these casitas's have shelved a lot of my higher end reels.
what i find is kind of funny is that when the new aldebaran came out last year i bought 2 at $350.00 plus dollars and absolutely hated them , although there were smooth they just felt cheap and plasticy ( i know this prob isn't even a word) but nobody was worried about them and i believe the reason is the marketing and price!! if shimano called the casitas something other than composite like molecular ci7+ or whatever and charged $349.00 for it people wouldn't be worried because it would be "high end". yes there are guys that won't even look at a reel that is not alum or mag framed as they are proven workhorses but coming from a background in the cycling industry ,composites / carbons etc are definitely here to stay and from companies like shimano with extensive engineering knowledge of these materials i think your going to see more and more out there