How likely is a revision conflict when using an update handler? Should I concern myself with conflict-handling code when writing a robust update function?

As described in Document Update Handlers, CouchDB 0.10 and later allows on-demand server-side document modification.

Update handlers can process non-JSON formats; but the other major features are these:

• An HTTP front-end to arbitrarily complex document modification code
• Similar code needn't be written for all possible clients—a DRY architecture
• Execution is faster and less likely to hit a revision conflict

I am unclear about the third point. Executing locally, the update handler will run much faster and with lower latency. But in situations with high contention, that does not guarantee a successful update. Or does the update handler guarantee a successful update?