It seems taking exponential time: O(4^n) by DFS solution.
Does this problem have polynomial time solution?
One can construct other exponential cases, e.g. nums="AdddBddCdddd..dddd" with even number of d's and target=A+B+C. In this case you'll have at least a set of strings where all d's evaluate to 0 and doing this with only '+'s and '-'s between d's will be exponential.
On the other hand there are special cases that are solvable in O(N). E.g. nums consisting of only even digits and an odd target or if nums consists of 0,3,6,9 and target is not divisible by 3.