Yeah, surrounding with opponent stones felt too strong of a downside for 1/11 (but not for 1/79 and 1/83).
Good point on 1/71 allowing you to bypass these downsides, maybe it should be changed so it still applies all the denominator effects but only lets you choose one numerator effect? That doesn't solve the 29/71 issue, though... but I'm of the opinion that 29 needs a nerf anyway (i.e. make it only usable once per turn, or make it only usable a few times total before it's discarded).
1/31 probably adds a copy of the same stone that you just played (albeit one that does not trigger its effects on play).