What are disadvantages of using foil method in algebra?

It just doesn't always work. You can only really use this method if you are finding the result when multiplying TWO dinomals. Well I mean it is possible to use it, but it will just be confusing, which was a problem I faced before.  
Personally, this method actually confused me, when I used a larger dinomals. There are other rules, not sure how many tho. But the other one you should consider, when working with larger dinomals, you should use "vertical multiplication" (idk if that's what you really call it). 

Hope this helped :) 

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction