Since the way chunks are created/saved etc. changed, chunkloaders don't work (properly?).
I tested it with chickenchunks and it creates chunkloader tickets, but the chunks aren't loaded. But no crashes either- that's good, isn't it?
This might be because the game (ticket-system, viewable via /chunkloaders in Chickenchunks) doesn't know which chunks on the y-axis to load?
Naively I would suggest loading the chunks above and below the chunkloader. Obviously not all of them! +/-8 would be vanilla (more or less) behavior or simply only the one CubicChunk where the loader is placed.
I've got no idea how hard it would be to implement this :/

Does it work with a cubicchunks save format (but normal height still, 0<y<255)? It may never load less than 0 with chicken chunks, but how about at, say, 64? And, are cubes chunkloaded individually?

In its current state Chickenchunks doesn't keep any chunks loaded. I just noticed that forge's ticket system registered the request to keep chunks loaded (/chunkloaders showed it). I tested it in the 0<y<255 range as well and it doesn't work there either.

Are cubes chunkloaded individually?

I choose to do the testing in the same x and z Coordinates, so only the 3 ranges "y in [-inf,0), [0,255], (255,inf]" could have an effect. Sadly no chunk outside if my character was loaded.

Tested via Minecarts passing a detector rail next to command block.

Aha - apparently this is a known issue (unless it was just added). If you go to the wiki compatibility list (https://github.com/OpenCubicChunks/CubicChunks/wiki/Compatibility-list) you can see it mentioned as "not working". I ran a quick search and couldn't find any other chunk loading mod. (EU2 has a chunk loading block, whatever that does, but EU2 doesn't seem to like CC)

I added that entry :)
Edit: Ftb utilities contains a chunkloading mechanic, but it seems to be broken in 1.12. Therefore I didn't test it with Cubic Chunks

I've made a cubic chunkloader mod, it's still alpha and it's not on github yet but the download is pinned in #general on the CC discord.

Mostly fixed. Many chunk loading mods still don't work correctly but it probably can be fixed. FtbUtils works. If there are issues with specific mods, report them separately now.