1. Security is the same.

2. Efficiency. Producing and transmitting Merkle proofs can become inefficient if the tree is too unbalanced.

On the other hand, an extremely unbalanced tree (where only only a single branch grows[1]) can be used in a hash-based signature scheme where it is expected that signatures are going to be very rare with likelihood of being singular - such that every signature bloats the size of the next one.

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1. Security is the same.

2. Efficiency. Producing and transmitting Merkle proofs can become inefficient if the tree is too unbalanced.

On the other hand, an extremely unbalanced tree (where only only a single branch grows[1]) can be used in a hash-based signature scheme where it is expected that signatures are going to be very rare with likelihood of being singular - such that every signature increases the size of all signatures that follow. If it's expected that a single signature is all that will ever be produced with the option to make more if necessary, it's a good (optimal?) scheme.

[1]

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