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Giving your team hexproof is a great roadblock for your opponents since they need two pieces of interaction to kill anything that's not Sigarda, but the card advantage is what you want

Force an attack Another way blue can deal with a creature is to force it to attack into something that will block and kill it

In general, I would look at these two factors when looking to include cards not of your tribe's creature type: Is this creature so strong that it, on its own, is generally stronger than my tribal creatures when they are receiving bonuses and/or empowering my other cards

Even so, it is a huge swing when it happens and that is generally up to you