Well, first things first, this army has the most beautiful and detailed minis you can currently find in the
GW range. Not to mention the minis will actually be worth something in 5+ years.
Don't expect to win a lot of games. It's a
very tactically challenging army.
On the positive side, these nuns can put an
insane amount of dakka/melta/templates on the board. And, well... that's all.
The Sisters are a sub-
MEQ force, with sub-par Strength, Leadership, Toughness, no
ATSKNF, no chapter tactics. And they are only marginally cheaper than vanilla marines. You can also see the as a über-guardsmen force, with BS4 and 3+ save.
The Emprah helps you if there's a Heldrake on the board...
The weapons choices revolve around the Holy Trinity : melta, bolta, flama. These weapons works well, but you really want to get close to your opponent. Unfortunately, being close involve
actually getting close, which can be done with your Rhino or footslogging. Good luck.
Not to mention being close involve sometimes being involved in actual close-combat. And as mentionned earlier, you've basically have guardsmen with a better ballistic skill and armour save.
This army works like any glass cannon : get close to your opponent. Kick him in the balls. If he's still standing, things will get ugly.