Thou shalt not mix
Sometimes a platform vendor has a set pattern, selection of tools, an IDE, or a prescribed workflow that has been shown to have a good success rate and is documented. Developers can sometimes take a moral stance and declare that they will not use anything that is outside of the provided package, with the intent that if it hasn’t been included then it’s not needed, will make it harder for others to join the project, is not supported by the vendor, or will cause complications later if upgrades are needed. There are many reasons why this stance may be taken.
Many of these are valid claims, and it really does depend on many factors. On small teams it can help to limit the number of technologies in use. On small projects it can help to reduce the complexity of the project for the same reason. It does seem to suggest that by preventing the inclusion of additional tools, frameworks, or libraries then we gain by reducing the amount of interactions, testing, integration tasks, and possible defects that can be caused by incompatibilities between these things. Resolving these problems can and often is non-trivial.
Some of the possible downsides include missing out on opportunities to use newer more refined techniques or technologies. There is a risk of upsetting staff when they are banned from using things that they feel offer great benefits. You may miss out on leveraging skills and knowledge within the team and this can hamper progress. Quite bluntly, the out-of-the-box method might not be the most productive, effective, reliable, or suitable depending on your metric for success.