Are there any known situations, although infrequent or difficult to meet, where the Stellar Consensus protocol could not work well or where the network can be attacked by malicious nodes?
A partial answer is here, but I'd like to know more: https://rubin.io/public/pdfs/oort.pdf
Two open problems in Stellar are the mechanism by which quorums are chosen (peer selection) and how new arguments may be proposed such that contention is low (i.e. avoid dueling proposers).
Stellar consensus can be extremely inefficient in terms of number of messages sent, especially with dueling proposers.