My main two concerns with Sexton are:
1. Will he emerge as a consistent shooter? Currently he's a work in progress and considering our other guard is a work in progress, I think it is problematic to pair two players that aren't shooters together in the back court. If Sexton and/or Ntilikina develop into shooters that is great news, but if they don't we would be relying on their defensive contributions, their ability to knock down shots closer to the basket and score at the rim.
2. Will he emerge as a premier passer? I don't see it. The player he reminds me of is Brandon Jennings. We could do worse than add a player with that ability because early in his career he was pretty good, but he was never really a premier passer, he was acceptable. He was also a chucker. On his night he could put up good numbers, but on his off nights he could cost his team games.
Maybe Sexton will be the next Donovan Mitchell. Sexton does offer a lot of promise with his drive, athleticism and quickness. His style could translate pretty well but ideally he would fit next to a shooting guard that can actually knock shots down at a good rate. Currently Ntilikina doesn't do that. Sexton doesn't either. So it's a risky pairing if neither emerge as shooters.