It can be a bit worse that that. If we get into an unlikely (but asynchronously reachable) situation, then there is an attack with less than 1/3 of votes that potentially stops anything from being finalised indefinitely. Suppose that in epochful Casper FFG with LMD on the last justified checkpoint and 10% malicious stake that votes late, we somehow have something like this:
A - B
30%- 60%
51% - 20%
Here the bad guys are the only missing votes on the first checkpoint. Now the A chain is winning under LMD with 51% of the vote so it will keep getting votes. But the bad guys can justify B and make that the fork choice at any time. What they can do is time things so that A gets around 60% of the vote at the time of the switch. If they pull this off, then B has the last justified checkpoint and is winning, but they have the power to justify A at any time. So they wait for the next checkpoint on B to have 60% of the vote and repeat.
Now actually any fork choice rule that builds on the last justified checkpoint is vulnerable to this kind of attack. If our situation is:
A -B
30%-70%
60%-30%
30%-60%
Then the bad guys who haven’t voted on the last two checkpoints can force a reorg to A and then later one to B by justifying these checkpoints.
We need to argue that either we never get into this kind of situation or that the timing is hard to pull off repeatedly. Now for epochless FFG, the timing of these kind of attacks can be much easier. If the honest guys in the 1/64 who vote on each block vote faster than a block, then the bad guys can reliably time to within a resolution of 1/64 of the votes. There are a few complications in replicating this attack on epochless FFG, such as the blocks that replace the checkpoints above actually being closer than 64 blocks apart but it should be doable with less than 1/3 of the vote.
So we are stuck for hoping that the set up never happens. I certainly have no idea of a credible attack that would give it.