Last I checked, they reject the Uncreated Divinity of our Lord and Savior Jesus Christ.
That in itself is a big enough reason to be concerned. The heavy cultish aspects do not help one bit either.
Once he joined the JWs he was literally a different person. Pretty much shunned me and all of his friends. Dropped out as a sophomore.
In retrospect, I realize he was searching for something. But he ended up thinking everything he did before was "evil". Gave up his beloved music, beer, etc.
I went to couple of Kingdom Hall meetings with him. Nice people, but sorry, quite strange.
They have "Communion" where they pass around cermonial bread and wine. The only people "allowed" to take it are members of the 144,000 elect. So nobody in the Hall partook. He told me that if someone tries to take Communion they often times are physically stopped, because taking the bread and wine is a grave sin if you are NOT one of the elect.
He also told me that Jesus was not THE son of God, however A son of God, an archangel. Gabriel? Who was re-incarnated.
I had many discussions with him after that. He broke off all contact with me soon thereafter.