The major advantage and disadvantage of a jury trial for an accused criminal are both rather similar. Juries are made up of lay people who are not trained in the law or in how to evaluate evidence or anything like that. They are amateurs in the field of criminal justice. This can be good or bad for a defendant.
The basic idea is that a jury might be swayed by things that would not influence a judge. A jury might be persuaded by an eloquent lawyer. It might be more sympathetic towards a defendant who is particularly good-looking or who acts in a certain way during the trial. They might be more likely to acquit based not on the evidence but on these extraneous factors.
However, this aspect of a jury is a double-edged sword. If a jury is capable of being excessively sympathetic to a defendant, it is also capable of being prejudiced against that person. Imagine if the jury is made up of conservative older people and the defendant is a young person with long hair, piercings, and visible tattoos. Imagine if the defendant does not know how to comport him or herself in court and seems disrespectful or disinterested. In such cases, the jury might turn against that person and be more likely to convict than is warranted on the basis of the evidence in the case.
Thus, both the major advantage and disadvantage of a jury for an accused person is that a jury may be swayed by factors that should not actually be considered when deciding that person’s guilt or innocence.