It's Sweeney's best UFA signing by far. Not to mention at the time, Jeremy Swayman had 11 NHL games under his belt and Rask's career was in jeopardy but not completely wrote off.
Credit the scouting they saw Ullmark for that horrible Buffalo team and despite how bad that team was, the games he played their record was markedly better than their overall. It's not like Sweeney went out and signed some established No.1 for a 7 year deal at 8-9 million. Ullmark was signed basically as a bridge between the Rask's era into the Swayman era, and in that regard he did the job, probably better than he was expected.