If the target of Rex Nebula's ability is a creature, it will lose all creature types (such as Zombie) it had until end of turn, and becoming crewed will not cause it to regain those subtypes
You really want to be curving out with early merfolk to ensure theres already an army ready when Hakbal arrives, and as a four-mana commander, you dont really benefit by ramping into him after turn two
If you target one creature with all three backups, it will get +3/+3 and trample, and if it deals combat damage to a player, you will double the counters three times
Us and the [Rules Committee] will keep an eye on them
In this environment, every point of damage matters, and the ability to convert early-game board presence into late-game reach is what separates a Tier 1 aggro list from the rest of the pack
It presents a lot of different deck-building options, and its flexible mana cost means you dont need all three of its colors to cast it