There is the usual problem where developers each make changes to the same component and these changes are, in some way, incompatible. However, where several components are being changed at the same time, the problems are exacerbated because there may be dependencies between the components that are affected by the changes. For example, say developer A checks out components X and Y and decides to implement a change by changing Y, which depends on a particular feature of X. Developer A checks X and Y back in with no changes recorded as being made to X. Developer B also is working on X and Y and changes both X and Y. However, the changes made to X mean that the assumptions made by Developer A no longer hold. However, incompatibility is not detected as there has only been a single change made to component X.
With more than 2 components, the problem becomes even worse because of the chains of dependencies that can be introduced. These can be very difficult or impossible to detect automatically