Well - whether-or-not it's actually true - it would be true to
slightly-more discourage the use of cards that create random effects.
Also, the quote from the OP is actually
"Random effects always have the effect that is most favorable to the opponent." .
(The difference matters when using search to
find that part of your post.)
When the effect
has a controller, I agree with your interpretation of
that quote, since that corresponds to when MtG text uses "an opponent" .
When there is no controller - such as a
"would be put into a graveyard from anywhere, ... shuffle it into its owners library instead."
card moving from a zone that's neither the battlefield or the stack,
while there was already 1 or more cards in that player's library -
I imagine you would say the owner of the card, and I am not aware of any current
cases in which that would differ from, player responsible for the effect's existence.
(A hypothetical permanent card with
"All cards in hands have [the replace-with-shuffle ability I referred to above]."
would cause those to sometimes differ.)
For
Donateing a
Mana Crypt, I'm giving 3 example decks
- albeit not ones which would be legal this season -
in case my explanation would otherwise be unclear.
ManaCrypt:
Island ,
Mana Crypt ,
Glacial Wall ,
Donate
MonoGreen:
Forest ,
Forest ,
Forest ,
Leatherback Baloth
BackBuild:
Veldt ,
Helix Pinnacle ,
Traproot Kami ,
Green Scarab
I agree with you that the result of the current rule is, MC beats both MG and BB.
For MC vs MG, I also agree that that is what the result should be, though for me,
that's because if a coin keeps getting flipped, the probability of
never losing 7 flips is zero.
For MC vs BB, I think MC should lose, even though the current rule has it win.