After all the gnashing of teeth earlier this week over my reactions to the allies rumors, I can finally react to the book itself.
On the FOC level, I don't see anything that's terribly exploitable.
For every primary detachment in your army, you can take one detachment of allies. Which is an HQ, up to 2 units of troops, and up to one of everything else.
It's possible, I think, to take 2 primary detachments and 2 allied detachments at 2000 points (Edit: It definitely Is). But aside from maybe one or two odd builds, I think that this will be reigned in by the cost of taking all the required units. So I'm not really very worried about it.
The Ally matrix chart is really hard to read. Who designed it? The thing looks like a game of Bejeweled. With an entirely too complex key of what shape/color/logo means what.
There are still 3 different levels of buddy, and most of the differences favor Imperials. So some armies will benefit from armies better than others, or not at all.
I won't re-hash what we've already argued about here. But I do think that TOs might find that they need to standardize the ally levels and/or availability across all armies. I think that would be much preferred to banning them, which I don't want to see happen.
I'm also afraid that while this will VASTLY increase the variety of forces in casual games (a good thing), that we'll find competitive players always taking one of 2-3 standard ally detachments (not so great). Maybe I'm wrong on that, but we'll see.
Extra detachments are another matter, and I really don't know what to do about them. If you ban them, Nids will suffer (unless they get allies too). If you don't ban them, everyone can pretty much take whatever they want.
Which is better? Dunno. Probably best to do nothing and see how much of a problem it is (or isn't) in a few months.