Would be that it would be

Is anything wrong with saying "My guess would be that it would be nice"?

I feel that the "would be that it would be" part sounds wrong somehow but I can't quite put my finger on why.

Would gives the modal aspect of uncertainty, so if you're sure of what your guess would be, but you're not sure of whether you'd like the situation, you'd say

My guess is that it would be nice.

If you're not even sure of what you'd guess, you'd say

My guess would be [if you forced me to guess] that it would be nice.

The reason the latter sounds a bit "wrong" is that a guess already asserts uncertainty.