1.Database Problem: If i add more sites in a single site collection, content database of that site collection will increase...
At one point of time database may be corrupted.
If there are any daily jobs which take database backup..it will effect because of the size...and restore also will get effected..
2.Groups: If we add more sites to one site collection then each site will have it's own groups...it will increase...site collection administrator will have tough time to manage all these groups..
3.Changes: If there is any change in the root level site permissions...it will effect to all sites...
If we change any content type of site column it will effect to all sites..
4.Recycle bin: Each site will have recycle bin and Site Collection have one recycle bin. If any site admin delete the data first it will go to site recycle bin...if site admin removes from site recycle bin, it will move to site collection recycle bin. At one point all deleted files will come to Site collection recycle bin, it will be very difficult to site collection administrator to handle all these...
Like this we will get more problems if we add more sites in single site collection. It will always best practice to create new site collections and add sites to that..
