I think part of the problem with AI is that Not all of the winners have acheived prolonged success; some of them don't even get a contract renewal.
I saw the Katy Perry movie this weekend with my nieces. Part of her story is that she was dropped by multiple companies who tried to mold her, instead of just letting her be Katy.
Due to America picking the winner, there are sometime incompatabilities between the winner and the recrding company that don't lead to success. However, due to AI's juggernaut status, it's as though if you don't get renewed then you must be a failure. Leading to another AI winner that failed to reach Clarkson or Underwood status. a Catch-22 scenario that tarnished the AI brand.
Also, some seasons are just 'weak' compared to the stand outs. I stopped watching AI years ago - Fantasia vs Diana and Taylor vs Katherine McPhee just didn't cut it for me. Since then, I've only tuned in when there's been big buzz about a stand out AI contestant. I tuned in a bit with Daughtry - one of the stronger seasons. Since then, I think I've only watched the AI finale with Adam Lambert. I also tuned in a bit to see how ST and JLo would do as judges and liked the renewed energy they brought to the panel.
For me, the judges need to be honest and provide constructive comments. I used to agree with Simon frequently, even though he was sometimes harsh, but I equally disliked when he was promoting his favorite, as well. However, I felt Simon's harshness was somewhat 'needed' because the industry and the public are tough - especially in the anonymous internet. What he often said was less harsh than what the general public will eventually say and contestants needs to develop a thick skin, as well as learn how to deal with negativity in public.