While MVCC works fine in the single-account case, it fails badly in cases where you want to maintain an invariant across accounts. For example, if a person can have two accounts at a bank with a total overdraft of no more than $100 (so A+B>-100), two MVCC transactions can alter one account each, checking for the invariant, and you still end up overdrawn beyond the limit. In general, the fact that MVCC only fully handles write-write hazards can cause many problems.
The whitepaper Serializable Snapshot Isolation talks more of dangerous read-write structures.
My MVCC may be vulnerable to write skew even though it generates the right result. I am yet to generate a test case that exhibits that behaviour. The write skew occurs when there's two dangerous read-write structures
