Leerans can read the thoughts of anyone within a few feet, so for security reasons they are banned from many worlds. This has led to several unsucessful discrimination lawsuits.
The Yeerk Empire has tried to infiltrate their planet and make them all Controllers, but fortunately the Leerans could sense them coming so they had to resort to a violent invasion in which all of their battle tactics were anticipated. The Yeerks have so far conquered very few Leerans, but those that have been captured make dangerous enemies and nearly divulged the Animorphs' secret, i.e. that they were not really Andalites.