Spectre on the hyperthreads
This was actually the vector of attack that designers were seriously concerned about fifteen years ago. It's a bit surprising to me that it has taken this long for this to come up.
Two threads per core ==> very easy to snoop on the other thread.
Four threads per core ==> much harder to know which thread you are snooping on.
Also, if an application says to the OS, "Please start this thread for me. I trust it to play nice with my main thread." I'm hard pressed to understand why the OS should say, "no".