Expectations are not met

The objective was to replace me the external flash with the Sigma 18-250 lens as standard in low-light moments. Unfortunately, the quality of yield is not a fixed focal length, what I expected. The autofocus is often wrong. In difficult lighting conditions tend to fail completely. The depth of field at f 4 shank my Sigma also, if not better.
On the whole, that which creates the fixed focal length from 4, the Sigma also creates, including making the external flash, the light of the weakness Sigma amends.
Unfortunately I have to continue with the huge lens and external flash work, unfortunately. But to pack for more than 330 a fixed focal length in the trunk, just so everything is a little more manageable, for me is the money too good, especially since it brings no discernible quality advantage. The Sigma does more and is also suitable for everyday use.