ddd, snake and dk's bair difficlut to sweetspot? are you kidding, its harder to land the whiffed part of the attack than the strong one
adult links fsmash hits harder with the second swing, which does kill quicker than sonics
pikachus dsmash is not used for kills, in fact, several characters use the dsmash as a move to knock people away, the only ones that use it for kills are DK, metaknight, C. falcon, lucas, diddy, wolf, lucario, and G&W, the rest are not strong enough to kill effectively and are used more as a clear or a special (zamus and snake) than a kill
sonics fsmash is considered a bad killing move because it comes out too slow for his playstyle, and has less range than slower fsmashes like dk, dedede, and ike.
another reason sonic is considered to have bad kill moves are the fact that his aerials are not good killers from the stage, and he cannot kill up top, diddy has similar problems, however, diddy has bananas that allow him toget off his dsmash and fsmash. and thefact that you left off the top killers and spikes out of the study points out another one of sonics flaws (horizontal kos are easy to recover from unless it is extremely powerful)