If an effect allows you to put a card with particular characteristics onto the battlefield without instructing you to play or cast it, you consider only the characteristics of a modal double-faced card's front face to see if that card qualifies
This kind of hints that these were tested for lower costs and are found to be a bit more busted than they ought to be for their rarities
Not just that, but for 2B, tap, and exile a creature from a graveyard, we can make a 2/2 black zombie creature token
It's unclear whether this was the most necessary of the Slivers, but it was the one that frustrated the most opposing plans to deal with the problem and even opened up new game plans when the card advantage resulting from saving your creatures could allow Sliver decks to go the control route when backed up by counters and removal spells
Even better, Losheel offers mono-white decks some extra card draw which is always a huge plus
And the maps are way less of a downside