from my experience tabligh jamaat will give the participator a desire to do good deeds but will not rid him of the hidden illnesses of the heart. usually people do not even recognise the illnesses that one has in his heart. it takes another person to point them out and who better than a person who is experienced in such a field and who has a special connection with Allah ? thats where tassawuf comes in. in my opinion both tassawuf and tabligh jamaat is needed and both will benefit inshallah. as for being lead astray, then that can happen both in tabligh and tassawuf. its just depends on the group one joins. there is good and bad in both. find the good in each and attach yourself to that, and always remember that you are in it for your own islah and not to do the islah of others.