I think the details of key relation doesn't really matter, be it HD-derived from a master key, be it derived in a chain from each other, or just a set of predefined keys, they all have in common:
- you have to create some state upfront, with secret and public part
- you have to safeguard the secret part separately from your 'day-to'day' nsec

These two points make pregenerated/HD schemes weaker.