Bullpen pitchers are notoriously variable in their performance from year to year (Just ask the Cardinals, who always seem to buy high on guys like Brett Cecil, Greg Holland and Luke Gregorson, and get burned). So the Brews, who were 5 games above their Pythagorean record in 2018, seem unlikely to repeat that formula. But they have good players and will be in the thick of it.
Yes, relievers are volatile, but Josh Hader is also better than any of those guys were, particularly at the points of their careers they were at when they were in STL.
The biggest threat to Milwaukee's bullpen is the fact that Hader, Burnes and Woodruff should really all be starting. Still leaves them with Jeffress, Knebel, Barnes, and others. Their staff is very flexible and quite deep, even if they don't have the established Cy Young-candidate ace at the top.
With the depth they have in their both their MLB roster and their minors, I'd expect Milwaukee to go all in on Jacob DeGrom if the Mets were to put him on the block. Burnes or Woodruff and Hiura would be a good way to start that conversation.