There are a lot of advantages, but since you asked about disadvantages, here are some:
possible lack of flexibility
A person who has successfully worked in numerous positions has demonstrated that they are able to contribute meaningfully under a variety of circumstances.
Chances are therefore high that the candidate will work well in our circumstances. In addition, in our agile business environment, circumstances might change, so we value flexibility.
possible lack of professional growth
A person that has worked with a variety of technologies has demonstrated that they can learn new technologies quickly. We are currently transitioning to a new technology stack, and need people who can get up to speed quickly.
possible ignorance of cultural norms
aka "bad cultural fit":
Everybody knows that people need to change companies often. Why doesn't he do that? He's weird. I don't want to work with weird people.
(Not a very mature reaction, but then not all hiring managers are very mature ;-) )
If a candidate remains in his position for very long, it can indicate weaknesses. A good interviewer will want to verify whether these weaknesses are present, for instance by inquiring whether the candidate has learned new technologies, switched teams, worked in different unofficial roles, or has demonstrated a capacity for growth and flexibility in other ways.
Let me stress again that this answer focuses on the negatives only because you asked for that. There are numerous positives as well, and rejecting a candidate simply because they have shown exceptional loyalty to their previous employer would be silly.