An excellent end cap to the expanded elementals. In this product we see all the previous elemental staples and variants presented at CR 10. All the elementals are well balances, though some come out deadlier than others by sheer difference in abilities and focus. This is not a detriment, but just a function of the elemental variance. That said, everything seems to hash out decently well, and the elementals are all nonetheless very dangerous and exciting as opponents.
One thing that I did notice is that there is a wild shift between given elementals in terms of armor class and hit points that MIGHT swing a given elemental higher or lower on CR than I am comfortable with, but this is definitely something that I can roll with and adjust at my disccretion in my games.
Each elemental is also given an additional attack form that targets multiple creatures (usually in a 10 foot radius) that seems to give the creatures a nice tactical option in battle that isn't just three slam attacks.
All in all, I love this product as it gives us just what we need in elementals at a high CR. If I had a request, it'd be that there be more exposition to these monsters, especially to expound on potential tactics and methods.
But really what I want is a compilation. All the books put together in a handy printable reference that collates elementals by type, then by CR.
4.5 stars, rounded up to 5.