If your opponent tries to destroy it again later that same turn, you won't be able to rely on the same card or ability a second time Regenerate Creates a Replacement Effect Regenerate is, at the end of the day, a replacement effect, which means it replaces one event with another
If you count these sagas as lands, then the Jeskai Chandra deck is half mana sources
This does not necessarily make those decks bad, as their power level can overcome this flaw if the combo it provides is simple enough to win games when it opens optimally
Meren has the ability to get them when stuff you control dies and note there are no once/turn or nontoken breaks there
Cavern of Souls It has been a minute since weve seen this in Standard, but it filled quite a number of big decks when it debuted in Avacyn Restored with blue-green spirits, green-white humans, even Naya Huntmaster of the Fells decks that used humans, wolves, and angels
Fenrir has been a few of those, first appearing as a summon in FFVI, but this version of his appearance as an Avatar in FFXI