Not sure about Horford and Rob Williams as a starting duo. I like them better as starter and replacement center with a few minutes alongside each other now and then.
I worry about the offense with both of them starting. Rob Williams is a non-shooter, above average passer but limited ball-handler. Horford (for a 2021 PF) is a reluctant shooter and therefore below average shooting threat, an average passer and a below average ball-handler (attacking the rim off the dribble).
I worry that this lack of offensive contribution from the PF and C position puts too much stress on the three perimeter slots. It requires Tatum and/or Jaylen to take big steps forward in terms of their playmaking for others. If they fail to do that and play more like they have done in the past, then the offense will become too stagnant and struggle to create open scoring attempts. This is further compounded by starting Smart at PG given his lack of dribble penetration.
I also worry about the lack of movement from a Horford & Rob Williams duo. Most teams nowadays have PFs that can move around the court. More active movement, more cutting. Horford is too stationary. He works well as the high post facilitator but not as a cutter or running off ball to get open shooting opportunities.
I believe we'll have to pick one to start and one to come off the bench in order to have a dynamic offense. With both starting, I fear the halfcourt offense will be too stagnant and easy to defend.