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This is done primarily to preserve system performance issues. Multiple nested/complex data/segments and criteria increase system load and degrade performance of your (and others) surveys.
Not really. This is a system imposed limitation and we are unlikely to change this.
Currently you can have maximum 10 groups at a time. If you need more Groups you can delete the Groups that are not required and create new Groups. Deleting the Groups does not affect the data in any way.