At least in the 4.0 branch, calling txnResources->release() in a ScopeGuard is problematic, since ScopeGuard runs its guard function in destructor context. In the 4.2 branch, this code was refactored and eliminated, so this problem only exists in 4.0.
https://github.com/mongodb/mongo/blob/715e61e81664d0e7872e91679a17666d80021cbf/src/mongo/db/ops/write_ops_exec.cpp#L137

If release() throws in this context, terminate() is called and the server will shut down uncleanly.