Tien's potential?

[SSJ2]

Doesn't mean much since the humans got better gains at Kami's in 1 year than Goku did in 3. Training boosts just get continually haxed if the plot demands it.

Well yeah. But it could be explained by the humans training together, which is shown to be more effective than training alone.

Goku sparred with Popo and Kami, no?

 

[Son Gokū]

Well, besides Uub, he's the strongest human I know.
Tien is a bad ass regardless of how high his ki is.

 

[withheldforprivacy]

Other thing: when Goku was about to drink the super Water, Karin told him that if he trains for several years he should be able to beat Piccolo Daimao. Tenshinhan, who was weaker than this Goku managed to surpass Piccolo Daimao in 3 years.

When Krillin first saw Jeice/Burter/Recoome, he said he'll never be able to reach that level. A few hours later, he'd greatly surpassed them.

 

[Let's Go Fearless!]

It's 's fault.