Why are alpha-carbonyl relatively less stable? So if we have an alpha halogen ketone, why is an Sn1 type mechanism so unfavorable for that species?

I don't understand. It seems like the C+ would be resonance stabilized by the carbonyl's double bond.
The carbon atom of the carbonyl group has a partial positive charge due to electron withdrawal by the electronegative oxygen atom. An $S_{N}1$ mechanism would result in an intermediate with a positive charge next to a partial positive charge, which would be high in energy.