On a lot of Famicom games the turbo just instantly broke the game. The one that comes to mind is Commando. With a turbo controller like the NES Advantage the dude's gun basically becomes a screen length sword that destroys anything. The reason it works so well in this game is because there is no specific limit for the number of shots on screen. It throws out as many as you can.
As turbo pads became more common, more and more games were made in such a way that it isn't even an advantage. Even Hudson's Soldier series, once the poster child for "How fast can you hit A?" eventually made turbo irrelevent.
IMO, Turbo is an essential part of Bonk.
Slow motion on the other hand. Yeah, its cheating. It's also basically torture in %99 of most games. Whether or not that cheating matters is another thing. In a high score competition, obviously it's unacceptable. Some games are just so f*cking hard though that I wouldn't hold it against anyone for using it. Transformers: Mystery of Convoy, for example. It's hard and it's bad. It's either going to get cheated on or it's just going to sit on a shelf.