I'm going to need to hear some logic behind this statement, because I'm not seeing it.
Assume there's 6 Rebirth minions on the Magmar's board at the time of casting.
You use all your mana, your whole turn to do 5 damage to everything, including your own general and creatures. Good everything on the opponents board is dead or is down 5 hp, but bad: so are you.
Final state: you paid 8 mana to (hopefully) clear the opponent's board and leave you with 6 eggs.
Next turn, your 6 surviving Rebirth minions can smack his ass!
The only problem with that reasoning is that the opponent gets a turn in between and all your minions are 0/1's. You basically gave your opponent a free turn while bringing yourself closer to death.
One Ghost lightning is all it takes to remove your Eggs. One spirit Harvester. Tiles of ShadowCeep, a Makantor, rush minions, etc... and that's with 6 Rebirth minions out.
The more realistic number is 2 Rebirth minions out. The enemy general walks up to one and smacks it, then casts , for example Deathgrip on the other Egg. They now have 8/9 mana to work with. They now have threats on the board after their turn and you don't.
Or maybe, while your minions are all Eggs and you put yourself 5 down for your turn, they go face and destroy your general. Possibly even with their minions that survived the 5 damage.
It can't even be used as a finisher for the enemy general, since Magmar have much cheaper ways to do 5 damage to them from wherever. (See Elucidator which even saves your General 1 health of damage. See Makantor, etc...)
I'd really love to hear your reasoning as to why this cards is not a bad card at its current cost and with its current effect.