The way I see it is second options. This time last year, you had Hader and Williams. If one of them goes down with an injury or (as what happened) ineffectiveness with Hader, you slide everybody up one slot. Williams was a perfect candidate to do that and late innings were still in lock-down mode.
This year, relying on Williams with more significant question marks behind him removes (or at least reduces) that depth.
Almost every bullpen is somewhat nerve-wracking going into the baseball season but I'm on the side of this Brewers bullpen being one of the things I'm most concerned about on the team. That and the nebulous outfield situation. I don't have to squint very hard to see that turning disastrous for awhile until the team can figure out which one of Frelick, Mitchell, or Wiemer is ready for big league pitching.