Pros - they get exposed to more material in a shorter amount of time. If they understand what they are learning they will be able to take more advanced math classes in HS.

Cons - the material moves FAST. Sometimes too fast for some kids. You might find them zooming through concepts like long division and not quite "getting" that solid understanding that they'll need in order to do well in subsequent math classes.

All kids are different. Some kids go into the regular math classes front loaded and get bored with the material. Some kids are just natural math whizzes and get bored with the pace in a regular math class. Some kids are decent math students but they need time to review and go over problems before they get a solid grasp of the concepts.

They can go up and down. They may quickly get long division and stall out and need extra support during fractions. Just be patient and watch for the signs of overload.