Completely open to collusion. All you need is a significantly large number of nodes accepting requests to be guarantors, and since there's no barrier to entry this is trivial.
The barriers are minimum coin holding requirements. So a cheater can't have many nodes there. Also, the whole idea is that most nodes are honest. If most nodes are bad, then you can't do anything whatsoever, and in which case the trusted centralized system is the only solution.
Moreover, guarantor doesn't do much, guarantor only gets involved if there's a disagreement between sender and mixer. If there's no disagreement (as should be in most cases), guarantor does not even participate the decision on distribution.