The intellect devourer consumes its host’s mind in a two step process
First it uses “Devour Intellect”
The intellect devourer targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 12 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll 3d6: If the total equals or exceeds the target’s Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence
This ability does not kill the potential host – but it does drop its intelligence to 0 and stun it. This is powerful indeed, but look what the Devourer has to do to accomplish it: have the host fail a save, and then another mechanic which will only go in favor of the Devour half the time against a target of average intelligence. Effectively two failed saves on the same turn. And to do this, it has to be within 10 feet of its host, when it has an AC of 12 and 21 hp (although it does have non-magic damage resistance).
Once the host is stunned (and thus, incapacitated) and has an intelligence of 0 (and thus, -5 on its Intelligence contest) the Devourer can employ its second ability, “Body Thief”
The intellect devourer initiates an Intelligence contest with an incapacitated humanoid within 5 feet of it. If it wins the contest, the intellect devourer magically consumes the target’s brain, teleports into the target’s skull, and takes control of the target’s body.
This second ability does kill the host’s mind, but it is unlikely to succeed unless the first ability has been used. Remember, to use this, the target must already be incapacitated. The Devourer has an Int mod of +1. Unless its potential host has also had its Int reduced, it doesn’t have a great chance to win the Intelligence contest.
Although the Devourer has Multiattack, it cannot use both Devour Intellect and Body Thief on the same turn. So we are talking about a two round process requiring three successful rolls and unable to be initiated unless the Devourer is within 10 feet. This is dangerous against a single opponent the Devourer manages to surprise. Against an alert party, even one of Level 2, the Devourer is unlikely to be able to finish the second ability, the one that actually kills the host.