I know she technically isn't canon, but how would you classify Android 21?
I'd figure she be like 17 and 18, but the whole Majin transformation threw me off.
I guess she would be a bio android, like Cell, but... I'm not too sure on that.
It depends.
She's created in the same way Cell was, but also has Dr. Gero's wife's mind built into her. If she actually has the woman's brain in her body, then...
uh...
"Bio-Cyborg"?
Yes I know. But unless they elaborate on latent human potential past what meager heights Krillin and Tien have achieved, the androids cyborgs are relying almost exclusively on their cybernetic enhancements for their power. Being able to train those parts of their body is ludicrous. It's like me saying my car will learn to go faster if I just keep driving it more.
So Super could have gone one of two ways and had it make sense:
1) Bulma offers to enhance 17's strength with her "I can make anything" BS and gives him a significant power boost before the ToP. 18 declines because she's comfortable where she is and wants to be a sparring partner for her hubby.
2) They go in depth with human potential in the way that the manga did with Gohan; explain that Gohan's unique potential is derived from his saiyan AND human sides, and that his ultimate form is when he's allowing his human power to coexist rather than be dominated by his saiyan power. If they better established that, then it would be more believable that 17's inorganic enhancements allowed him to bypass mortal limitations and draw out a power similar to Gohan's, but only if he had a greater external force to work against.
Instead we got, "I've been doing pushups alone on an island full of animals for years and now I can spar with gods."
I think there's actually an argument to be made that Humans have higher potential than Saiyans.
Look at it this way, Krillin, Yamcha and Tien saw better benefits from their Training with Kami and possibly their training with King Kai (in the case of Yamcha and Tien), though by that point Goku had Super Saiyan so they were entirely irrelevent.
Not only that, the part of Gohan, Trunks and Goten that makes them more powerful than their fathers seems to be their human halves.
This is all head-canon, though, so... *shrug*