Echidna's con is 9, I wouldn't call that "low", and even if they can double, if they got strength screwed, it wouldn't matter if they attacked twice or not.
And even if Shanna can double something, if her strength was terrible, it wouldn't matter, you might say that giving her a stronger lance would help, while that is true, it only can help so much if she's never getting strength, I think other FE games handled the whole weight thing much better.