Direct, private, and strictly point-to-point.
If A peers with B, and B peers with C, A cannot reach C through B. Ever.
VPC APeered with B
VPC BPeered with A and C
VPC CUnreachable from A
Why pairwise peering breaks down
10 VPCs45 connections
20 VPCs190 connections
That combinatorial explosion, not a feature gap, is the real reason Transit Gateway exists.