Restricting any unit for a game like 40K doesn't work because, unlike something like MTG, you have a very limited amount of resources to make an army out of. Restricting cards in MTG doesn't mean asmuch because there are thousands of cards you can still construct a deck from, in 40K and FB you a limited to the 20 or so units presented in the codex. Restricting that one unit could hurt your codex quite a bit.
Well given that the ally matrix allows everyone to ally with at least two other armies barring Tyranids, that might not be entirely correct. Consider:
In any given MTG tournament, you are restricted to a selection of cards from the many thousands available (let's assume you're playing Standard Constructed). Some of those may be banned. Most of them will not work in your deck archetype (almost all aggro cards are useless for a control deck, for example), and anywhere between half and two thirds will not fit in because of colour restrictions (with tri-colour being the most you can realistically build a tournament worthy deck from). Of those cards that match both colour and archetype that are currently in Standard, something in the region of 60% of those will be jank that have no place in a tournament deck.
Thus your pool of "potentially useful" cards that you can consider adding to your deck is often in the low tens, rather than hundreds range.
Compare that to the Space Marine codex, which has (at last count) 54 individual units in it, and suddenly the comparison doesn't look so wildly different, especially when you add in the fact that they are Battle Brothers with Blood Angels (41), Space Wolves (33), Dark Angels (34), Imperial Guard (45) and Tau (25) to give a grand total of 232 seperate units (not including upgrade characters like Telion) and that's not even considering Allies of Convenience/Desperation.
Obviously there is some overlap between codices (Tac squads are basically the same, no matter which book they come from) and there are also some terrible units in each codex, but we haven't even looked at Fortifications, Forgeworld or Apocalypse yet.
Therefore, even if some tournaments end up banning certain units (so far we've got a list of, what, 3-4 units and a wargear item?) there are *plenty* still left to choose from.