I'm not really happy with how these seem to be working... or not working as the case may be.
When fighting critters (PvE) sometimes they whack you with a melee attack and you get the "Held" bar... but Motion Accelerator doesn't seem to affect it. This also seems to happen in PvP. Several times I have been "Chain mezzed" in PvP, which is a bit contrary to the "Immunity" Motion Accelerator is supposed to grant.
While I have no actual idea how "Holds" are set up in this game, it almost looks like Motion Accelerator offers a "Magnitude 1" protection from Holds. However when holds are stacked, they overcome it. The Melee Attack hold may not even be a "hold" but some other status effect that is incorrectly using the "Held" UI. This may or may not be intentional, but it is misleading to say it offers blanket 'Immunity" when it really is a limited immunity.
To a certain extent this applies to Starship Mezzing as well, the Hold protection seems ablative: after the first hold is broken free from any additional ones are unaffected.
The lack of a status effect/debuff display on the Player's UI doesn't help evaluation of this, or any learning curve of what hit you and how to protect yourself from it in future.
Many things on ground combat don't work at all. A bunch of the ground traits are broken too, stubborn doesn't seem to offer anything vs roots/slow/hold despite its description. Just bug report it and forget about it.