I've said this before elsewhere, but....
My problem with Planescape is that what was presented wasn't particularly well fleshed out. The art was amazing and it had good ideas, but....
- The factions were presented as having ideologies but not as having goals. This kind of left them as nebulous religions without deities more than living organizations. They feel like chess pieces that you don't know the rules for moving or capturing.
- Too many of the factions feel too similar, as well. There isn't quite enough identity to each faction, IMO. I don't feel like the Faction War helped anything, either, even if we set aside that it wasn't well received.
- The Lady of Pain is a terrible NPC. She's iconic. She's memorable. She's mysterious. And the DM and players are told that PCs should never ever interact with the Lady of Pain. That is an awful NPC.
Now, I know some people like the above choices. I don't. To me it feels like a campaign setting under glass. Like a ship in a bottle, it's meant to be looked at and admired rather than played with as a toy. And sure, you could use Sigil like a city-sized World Serpent Inn, but then you're kind of ignoring all this world building that makes Sigil interesting.
My other complaint is that the nature of portals to other planes feels like the DM has direct control and the PCs have less agency. Like portals can be anywhere and go anywhere, but where they go isn't up to the PCs. And they're not necessarily two-way portals, even. That takes a lot of control away from the PCs in ways I don't really like.
All in all, it feels very easy to ignore the world building that makes the setting unique or interesting. Almost to the extent that you're supposed
to ignore all the lore and backstory of the setting. It's just a strange choice.