I think his voice is sometimes overrated not his compositions. His past compositions were awesome (from Tristania to Sirenia's past music) but he is overrated at all because in Tristania he wasn't the only who composed that great music as always people say.
He's not overrated, in fact my favorite albums of Tristania are the ones with him in them. Sirenia was awesome too until their pop experimentation on their latest album(which is probably his only bad album so far).
He sings well, his lyrics are great and he plays well too.