This level shows that even the best level designers started out with shitty SMB3 Wood levels.
Rainbowblock Way Review more like beatdown
Gfx: eh/10 (translates to 2/10)
There were the rainbow blocks, but I really didn't like them at all...
Also SMB3 recolors of some SMB1 BGOs that didn't fit into the level and looked just sooo out of place.
Music: 3/10
Not the best music for this level. When I hear Gloam Valley I think "cheerful, but also REALLY annoying", not a good choice for a wood level anyway.
The underground music:
why
After hearing Gloam Valley I also expected Super Paper Mario underground music or something along the lines but not
this.
NPCs: 3/10
There were about a bijillion different enemies in this level, and none of them except for the tower-goombas were interesting.
Also, talking about the tower-goombas

Did you really intend for
this to happen?
The magic word is
playtesting
There were power-ups... sometimes... but not enough for a level that is
allegedly supposed to be in the first world of your episode.
Gameplay: 1/10
If I had to honestly describe how the gameplay in this level was I'd use something along the lines of "meh" or "idk" because you walk down the rainbow brick road for a few minutes, stomp on a few enemies, find a P-Switch if you search for it long enough and then the level's over.
This was, even for a SMB3 Wood level, pretty boring. Nothing made this level memorable or at least a bit interesting. Most parts were really random (after the midway point 2 block wide ?-Blocks appear, which serve really no purpose at all that differs them from the normal ?-Blocks. Also sometimes there were random pipes with water coming out of them, for WHatEVer reason)
And the replay value of this level is almost 0%
I, for one, am glad that Zlaker makes better levels now 
The only purpose this level really serves is to show that even the best level designers started out small. 2.25/10, moved to the Trashcan.