I'm starting a Horus Heresy army this Christmas and it brings me a new problem, as I'm the first of my group to do so, so could I play my new Sons of Horus against the usual 40k battles?
The internet isn't much help in here, as there are as many people that say yes than no, an after some time searching I'm even more troubled, so I read the rules, compared them to existing armies and made some studies and even though the heresy legions can seem in advantage because all the D weapons, super heavies and primarchs (oh my god the primarchs), they have a great flaw in the actual 40k game, they need to have big units so they aren't flexible at all. For example, ten tactical marines cost 150 points, which is overpriced, while taking more costs just 10 points per miniature, wich means that taking ten more costs just a 100 points, which is really cheap, meaning that to have balanced units they should be as big as possible which totally destroys the usual flexibility of actual space marines and thus, the legions work in a totally different way, with few powerful units, a system that doesn't work to well in seventh edition with the tactical objectives.
So, in conclusion, even though the usual 40k armies will have some problems at getting down some units from the legions, they will easily out-maneuver them, so I think they are pretty balanced, being the legions more powerful but less flexible than other armies.
I will be testing my Sons of Horus soon to confirm all of this, and I will post my new ideas about them, as well as some pics.
I hope this article was useful for you readers and I'll be posting something soon!