Its almost always used as a secondary creature type in conjunction with a more popular one (vampire horror, for example), though there are a few stand-alone horrors
As proof of concept, Aaron turned in a card with his mechanic proposal
Most frusturatingly, perhaps, the game had a serious draw support issue, with unequal access to a strong draw engine across its several ink types
While values change, this could be due to the viability of the card
They don't copy whether that creature was tapped or untapped, whether it had any counters on it, whether it had any Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on
You can draw tons of cards, go wide and gradually buff your board, or make Kumena unblockable and try to knock a player out with commander damage