Tacullu's Mind Capture is not a "straightforward strong" ability, compared to other summoners' abilities, rather it is deceptively / indirectly strong. The true power of it isn’t that much economically, but rather positionally.
Tacullu's Mind Capture can turn a unit in a good position for the opponent into a GREAT position for you.
Enemy units in front of their gates ... now blocking that gate.
Enemy units protecting their summoner ... now becoming their assassin.
Enemy units threatening your summoner ... now protecting your summoner.
Mind Capture also allows you to get an attack in where you wouldn’t otherwise, and in general disrupt formations as Breakers love to do.
It’s not an economic ability, in general if the game enters a slow state you are better off just killing the unit, but if the game is at a moderate or fast pace, Mind Capture allows you to gain tempo and positional advantages very cheaply.
Mind Capture is both an offensive and defensive ability. Mind Capture on offense helps to convert enemy units into personal assassins or gate blockers.
Mind Capture on defense can allow you to turn the tables, Tacullu instead of being surrounded by enemies can suddenly be surrounded by allies.
Don’t be afraid to position Tacullu very aggressively, Mind Capture can help you be pretty safe and aggressive positioning increases the threat of Mind Control.