if you know how to use bloodseeker its actually easy. althought not support hero but a carry hero. but mostly he's advantage is the ability to restore HP and stop an opponent from moving once he cast a rupture (last skill). His other advantage is also called blood thirst. So once any of the opponents hero's HP is lower, it automatically boost up his MS and AD at about 75% of the opponent's HP when about 25% BS will gain true sight on that opponent. and he is also very useful in farming Neutral creeps.