Flamers are good against horde type (like Tyranid) armies because they use a template to determine how many hits are scored. Against horde units, the template will hit a lot of models, that's good. They also have an AP (armor pircing) value that will beat Tyranid troops armor, so they bug will have no armor save or cover save (template weapons ignore cover), so you roll to wound the number hits you were able to get and take that many models off the table. so yes flamers are good against Tyranid but work no differently against them.