Bastiaan is correct - Surface Automation if built in accordance with training should be just as reliable as any other interface. From my experience unreliable SA interfaces when looked at will totally disregard a number of key lessons such as image recognition, tiny waits after activates or focus events, etc. Poor interfaces will only use blind coordinates, good interfaces will use image recognition to find elements.
I know of one SA interface at a client which was running for many years - as reliable as any other interface.
For speed - it is slow, especially for new users. One partner was taking 40 days per process to delivery but that was cut down to 7 days once they had got used to using Surface Automation and were designing their solutions well to be able to gain from object re-use. V6 is a big improvement because a lot of the best practices many developers used to ignore are now built into the product.