This is the issue with Zion, you may be getting an all star or even all NBA player, or you may be getting the next Ben Simmons. Either end of this extreme is very reasonably possible. BOS management can assess this, with consideration as to whether his weight or attitude/maturity is likely to be better or worse going forward, but this uncertainty represents risk.
There is risk with Murphy also, but less. Murphy could continue to improve and maybe be an all star, or he could be exposed as a stat compiler on a bad team. His floor-ceiling range is not nearly as extreme as Zion's, but there is risk and uncertainty with him also.
Then you look at Brown, all star, 2nd team all NBA, NBA finals MVP, League MVP vote-getter, his floor-ceiling range is pretty certain. Not much risk at all. You pretty much know what you are getting with him.