But only if paired with Voxelgarden (which Unter Null was originally designed for), because VG provides a lot of the bootstrap mechanics a skyblock needs by default. The limited MTG support in this release is not very good(1) and has been removed from the current git version (that also changes lava cooling to be depth based which adds better progress).
So what does this mod add to the base game when used with Voxelgarden:
a world consisting of a single large ocean
a chest with some stones, some papyrus roots, a bit of dirt, and some saplings
the recipe for a (not very polished) grinding machine
lava cooling changed to sometimes give random ore nodes from flowing lava
recipes to create lava
Coupled with various gameplay mechanics of Voxelgarden this is (actually more than) enough to bootstrap one's own world from nearly nothing. If - like me - one doesn't know Voxelgarden yet, it also adds the fun of discovering the various neat mechanics that Voxelgarden provides for this process and the ways it differs from MTG (dirt is a falling node!!! but no need to restart, it is easy to get more dirt).
Also it is great that falling off is not fatal like in a typical skyblock.
Currently there are downsides:
Voxelgarden doesn't have any middle to late game content (like MTG), so one will need e.g. automation mods to keep having fun after the initial buildup; note that not all MTG mods are compatible with Voxelgarden
the lava cooling is a bit too easy in the released version, checking out the depth based lava cooling from git is recommended (even though building under water is a bit grindy in Voxelgarden)
there is a lot of polish missing, e.g. the generation of the inital chest may fail, and the UI of the grinder is primitive without any support for shift clicking
(1) the MTG support adds the papyrus roots to be used as a building and plant base in the ocean, but a lot of nodes are not creatable, e.g. most plants
But only if paired with Voxelgarden (which Unter Null was originally designed for), because VG provides a lot of the bootstrap mechanics a skyblock needs by default. The limited MTG support in this release is not very good(1) and has been removed from the current git version (that also changes lava cooling to be depth based which adds better progress).
So what does this mod add to the base game when used with Voxelgarden:
Coupled with various gameplay mechanics of Voxelgarden this is (actually more than) enough to bootstrap one's own world from nearly nothing. If - like me - one doesn't know Voxelgarden yet, it also adds the fun of discovering the various neat mechanics that Voxelgarden provides for this process and the ways it differs from MTG (dirt is a falling node!!! but no need to restart, it is easy to get more dirt).
Also it is great that falling off is not fatal like in a typical skyblock.
Currently there are downsides:
(1) the MTG support adds the papyrus roots to be used as a building and plant base in the ocean, but a lot of nodes are not creatable, e.g. most plants