Interesting question and answer.
I thought the answer should have been something not listed at all. Surely the big disadvantage is the failure to interact sensibly with Failover Cluster Instances: no automatic recovery, has to be manual; Add Dastabase and Add Replica don't support proper initialisation for secondary databases, so this has to be ground out manually in advance of joining the secondaries to the FCI; and changing the network name of an instance looks like a right pain. That was not an option, so I couldn't make that my answer. The other thing I though of was price - supported only by Enterprise Edition with the new 2012 licensing arrangement for SQL Server will put this outside the price ranmge of a lot of companies; but that wasn't one of teh options offered either. So I picked one of the options that were offered at random (got it wrong: the odds were 3 to 1 that I would - or actually 2 to 1, since multi-database failure is and advantage not a disadvantage so my pick wasn't quite at random) so that I could see the answer.
I'm not impressed by the answer. I think that both the disadvantages I mentioned above are going to be a pain for far more people than the one given as the answer here. Sure enough in the short term the restriction to one domain is a potential pain for people migrating from a two-domain mirroring system, but I can't imagine it being as much long term pain as the enormous price boost or the screw-up of the interaction with FCIs.
edit: I see that the reference given in the explanation doesn't actually say that the single domain restriction is the biggest disadvantage; it actually says it's one of the biggest disadvantages.