Disadvantages
A disadvantage of group interviews is that sometimes, one or two participants may dominate discussions, rather than allowing everyone an equal chance to effectively voice their opinions, thus limiting the myriad of opinions and contributions and ideas the interviewer may receive.
Moreover, peer pressure to conform to a groups ideas and opinions may limit people's ability to voice what they really think, making this method invalid.
Also, as discussions naturally do, getting distracted and side-tracked is common, thus the researcher must have a good ability to redirect the conversation back to the topic - otherwise, they will not be able to get relevant information.