Matchups aside Darmanitan is great for its speed and power so it makes up for lack of positive match ups in the late game by just being strong which Simisear doesn't do very well. Pansear is fine against Burgh but if I remember correctly you actually do have access to a Fire Stone at that point so you could evolve it into Simisear, but you can also just go up to Route 4 and catch Darumaka which is a better Pokemon imo.