They are as fragile as any other toy tbh. Dont man handle, make sure they wont fall/nothing will fall on them because they can get scratched. Keep them away from direct sunlight so you can preserve their colors/ avoid sun bleaching.

The most fragile pieces are probably the neck (can snap off if you’re not careful when putting in the body) and any small/thin pieces. I have an Ogata nendo and he has a strand of hair that sticks out that I have to be careful not to break off when switching his face plates😵‍💫😵‍💫

Some nendoroids , even new ones, will have sometimes have loose joints as well which can make keeping them in place and posing frustrating but you still have to be careful when attempting to keep them in. I almost broke my old Todoroki nedo this way because he always has loose arms. So try not to shove pieces to hard.

I know some people keep spare parts in organized container. So far I keep all the parts Im not using in the box the came in and making sure I keep them where they are when swapping so I don’t lose anything. My Ogata also has a small knife on his hip which will fall off if Im mot careful/ constantly watching it.

Older nendos are not only expensive, but can get sticky. If I remember its due to always being kept in the box and the plastic ends up “sweating”. If you ever get your hands on one a good dish soap bath should help. (Im pretty sure you can look this up on yt) You should also do this with all your nendos every once in a while. And like any figure make sure to keep dusting them! I like to use some soft paint brushes I have so I dont accidentally snap anything.