They are the stronger DSP by a distance. The gap they leave is UK-specific paperwork and visible approximation.
You want a proven programmatic desk today, with live supply, support and a service team, and your channel set is the one a DSP already covers. That describes most buyers, and StackAdapt is the better answer for them right now.
Your plan has to include UK cinema or addressable TV alongside programmatic, or you need the approximation in your targeting written down and auditable rather than assumed.