Snakes! They will sneak in at you, they will eat all your leaves, all your peat, and you'll be horrored! Cut them (violently) to get cobble. They also poop lode.
(Seriously, they are very scary and this mod is very violent)
I run this mod on a family private server (where the gameplay is less "hardcore" than vanilla) and it's a nice and very natural-feeling extension of gameplay. It changes the gameplay balance, especially making ongoing lode supplies much cheaper, but compensates nicely by adding an engaging snake feeding and caretaking system.
The snake's pathfinding is really dumb, sometimes pathologically so. Snakes often naively trap themselves, even with plenty space to roam, and while this makes it easier to "hibernate" them to keep them from running out of food, it makes managing snakes sleep/wake status and food consumption more about periodic annoyance than puzzle solving and careful planning.
The snakes seem too rare, especially on small maps that NodeCore otherwise makes very viable.
Snake heads are unpredictable, blend in with the body, and delicate, to the point where it can lead to "cheap shots" where the snake effectively suicides itself by running into your swinging pick while you're trying to expand its enclosure, and the cost of killing a snake can be catastrophic.
UPDATE: I'm revising my criticism, because, at least since the addition of Forms to the game, it is highly feasible to catch a snake and trap it in an enclosure that ensures it stays safe and healthy indefinitely, as long as the player remembers to keep it fed, which is not an error-prone process. Leaving design of such an enclosure as the player's responsibility is consistent with the challenge level of NodeCore.
In addition to less self-trapping behavior, richer pathfinding interactions might be nice, like certain kinds of nodes it avoids (igniters, radioactives, metals?)
Reproduction/renewal mechanics, such as laying eggs if fed certain high-value resources. It would be nice to be able to keep dormant eggs in storage until needed, and then incubate them somehow to hatch them, esp. as mitigation for snake fragility.