Mystcraft BoP Disabler is a very small mod meant to "fix" the current incompatibility between Mystcraft and Biomes o' Plenty for Minecraft 1.12. If you have both of these mods installed, Biomes o' Plenty biomes will spawn correctly in vanilla dimensions, but they will come out undecorated in Mystcraft dimensions. This mod disables the Biomes o' Plenty biomes in Mystcraft's symbols.cfg so that they do not spawn in Mystcraft dimensions.
In order for this mod to work, Minecraft must be completely started once with both Mystcraft and Biomes o' Plenty installed so that the symbols.cfg file can be populated by Mystcraft. If you are starting Minecraft for the first time with all three mods, you will need to restart for this mod to do its thing.
If, at some future time, the Mystcraft/BoP incompatibility is fixed, you can re-enable all of the BoP biomes through the Mod Options menu or the config file (mystcraftbopdisabler.cfg).
Be sure to check out these other great Mystcraft add-ons: More Mystcraft, Linking Through the Ages, Pages, linking-tweaks, and mystcraft-info.
I think things have changed somewhat, though BOP and Mystcraft are still incompatible.
BOP biomes are not generating as flat for me when I have BOP and Mystcraft together, but none of BOP's trees or plants are generating. So you have cherry blossom groves with no cherry trees, seasonal forests with no autumn trees, lavender fields with no lavender flowers, etc.
Results will vary, but yes, the biomes will be undecorated. I'll reword the description. Thanks!