It really depends more on what the minions can actually do. There are some skeleton minions that have ranged attacks, which put them well ahead of melee only minions. Also consider that attempting to create a minion by following the instructions in the DMG / MM (I cannot remember which book has it), can have some odd effects. All the pre-statted minions have static damage ranges and they generally have no special abilities. But if you take a random level 6 or 7 Brute or Soldier, and give it 1 hp, but leave the rest in place, the resulting minion will have an unfixed damage range.
And what happens if the base monsters had an ability that triggered when bloodied? How does losing that affect the monsters effective level?
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