Author here. Point is middlewares that are not built from ground up to scale horizontally are bound to die. BizTalk uses SQL Server for its storage as such cannot be horizontally scaled. End of story. NServiceBus... well, let's not go there :)
It basically a message based event driven transport layer. By its very nature it's extremely easy to scale horizontally.
For HA you simply run a distributor process for each logical event on windows cluster and add as many worker nodes as you see fit.
Because NServiceBus uses the store and forward pattern if a NServiceBus process/machine hosting it goes down, you are still guaranteed the eventual delivery of messages when the process/machine is resumed.