I find that why they do synergize well, I rarely get as much value from them as I need to given the time/mana investment. The necromancer, will completely negate the pestilence. The warlock, beastmaster and forcemaster will often be rushing you more or less solo and you will be off to a slow start, a bad place to be up against a rush. The wizard too will often have few creatures and at least in my deck nearly all of them are immune to pestilence as well. You will probably have a decent time against the priestess though!