Overall, I think that the Celtics have been very good at drafting in recent years. Of course you can miss out on someone but nothing says that the player would have been that good in your team, your system etc. Look how many franchises Butler went through before finding his spot in Miami...
I've seen someone post recently on Twitter the number of playoffs wins for players who had been drafter in the top 10 in the recent years : Marcus Smart has 28 wins, the rest of the top 10 combines for 26
https://twitter.com/Timi_093/status/1304803010737065985He did the same for Brown and Tatum and the result is pretty much the same. So I think C's have been very good at building the team, especially from a defensive perspective.
As for Giannis, I would agree that NBA tends to overlook european players, but it might be because I'm from Europe