In my experience, it's been the "tough" karateka and sometime jujutsuka that say Aikido is ineffective... My only guess is that they look, see someone attacking, see that same person flying, and don't understand why. If they don't understand why, it must be fake. The myths about Aikido being fake have been around since Americans saw the videos of O'sensei tossing people around a foot taller and a good bit heavier than he... "How he's doing that? He's just a feeble old man... it must be all show."
Etc. Etc... Come train with a few of my sensei, and tell me it doesn't work...