No heavily armoured infantry.
High points cost.
Usually very good magic. A good idea to beat them is to take a fairly large regiment of fast skirmishers with the Ruby Chalice to kill of the RBT's. Your own missiles and war machines will suffer from the Tempest though and it's easy to dispel. All the usual comments for dealing with shooting apply. Chaos can also nab off the Tempest without having to worry about their own missile weapons and war machines.