OK, I finished my first read of MEG: Nightstalkers.
Alten's latest is full of creature action, indeed a little overstuffed, and the chemosynthetic ecosystems presented would have to be orders of magnitude more productive than known examples to support a food chain allowing for giant-predator populations. (I'll give him a pass on the last such ecosystem we visit, though, because he's conjured up some invertebrate predators that may be unrealistic but are among the creepiest, scariest things in all of monster fiction). There is also a science fiction thread that makes sense only if you've read his novel Vostok.
I always nitpick mistakes in technology in fiction. I recently worked on whale-tracking technology, and the tech to track a whale 13,000 feet down, or under an ice sheet, in real time (as opposed to using pop-up tags that archive data) is too much to ask: marine tracking devices have very low power, under 1 watt, because otherwise they'd drain the batteries too fast. A character says at one point (correctly) that they can't track a whale in a subterranrean river deep under an ice sheet, then says on p.318 that they can.
The old characters are all here, of course, and the game of "who will get eaten?" remains suspenseful to the end. Overall, the characterization is a bit below that in Alten's best novels, Sharkman and The Loch.
There's kind of an arms race between Alten and Max Hawthorne over who can write the biggest creatures. Hawthorne's latest novel takes a a funny poke at Alten's "puny" Megalodons, and Alten pushes back by making his Liopleurodon not only bigger than Hawthorne's but saying it surprised scientists because the fossil evidence is for creatures only half as large. Hawthorne has been arguing that one fossil supports a gigantic pliosaur, so this could be a dig on Alten's part. I'm not sure whether this is deliberate, but it's a fun connection to make. In general, I have the same criticism of Alten and Hawthorne I always do: even if you set side the implausible circumstances needed for undiscovered survival up the present day, I think the creatures are too big, too smart, and too emotional. But if all you want to do is enjoy the adventure, you can set that aside as being within the purview of a novelist and go with it.
In sum, this is a crowd-pleaser for Alten (and Hawthorne) fans: exotic locations, giant predators, lots of blood, and a mix of heroic and stupid characters (who are sometimes the same person) getting in and out of hair's-breadth scrapes. Alten turns his fondness for pop culture and reality TV up to 11, and that's always fun, and my favorite predator of all time, Dunkleosteus, gets a couple of pages. If what you want is a slam-bang, ocean-spanning monster adventure, you'll enjoy diving into this one.