The broad concern that some people have is misplaced (China doesn't care about the average American home). The narrow concern is extremely plausible: that China would happily use it to target dissidents for example, or people that have fled China for various reasons. We've seen how aggressive they are over time in targeting those people, including physical kidnappings in the US and elsewhere.

The acquisition of iRobot should be immediately blocked on national security concerns. China would have no problem doing the same if the situations were reversed.