Brooklyn will play Seth Curry, Patty Mills, Goran Dragic, and Kyrie as their guard rotation with Drummond at the 5. That’s 3/5 of their lineup with a below average defender to take advantage of or have to consistently cover for.
They were more switchable in the playoffs last season.
They played Nic Claxton and Blake Griffin at center. Nic Claxton is a quick footed undersized center who plays very good team D. Blake Griffin is a forward who is quick for a center and a good switchable player for a center. They have shifted away from this to playing LaMarcus Aldridge and Andre Drummond at center. Both are slow footed. Both are poor at switching onto smaller players. This has hurt their switchability at center.
Then, like you noted, all the small guards; Patty Mills, Kyrie Irving and Seth Curry. All 6-0, 6-1, 6-2. All struggle on switches due to their lack of size. None are tough physical small guards like a Lowry or a CP3. Curry and Mills are both easily pushed around. Kyrie is their most able defensive guard - a scary thought. This hurts their switchability in the backcourt.
So now you can't switch at either guard spot or your center spot. That is 3/5ths of your lineup.
And it is not like they are blessed with talent at forward either outside of Kevin Durant. You have the extremely limited Bruce Brown. You have lost Joe Harris to injury. You lost Jeff Green in the offseason. Harden who was big enough to play SF has been traded. Next up is Bembry or the rookie Edwards. There just isn't talent here. It is Durant ... then Bruce Brown ... then Bembry, Edwards.
This is not a good basketball team.
Not without Ben Simmons. They need Simmons to make this team come together and be a high level basketball team again. Whether that is to replace Harden or to replace previous players Jarrett Allen and Caris LeVert. Without Ben Simmons, they are seriously lacking in quality outside of Durant & Kyrie.