Major Incident disadvantages

Answer:
You noticed correct - high pressure (including a lot of buzz and mess inside the incident management team are example of disadvantages. The other, what I can think of, are:
- misuse - when there is no right definition of what major incident is - there is a danger in misusing incident categorization as major incident in order to get more attention, resources and quicker resolution
- costs - resolving major incidents will require recruiting of best people you can find. Sometimes external ones. And such resources are expensive. More incidents categorized as "major" - more money spent.
- time factor - major incident gets (usually)highest priority, so all other activities are paused until major incident is resolved.
But, please don't take disadvantages as stopper for major incident process implementation. Such process has to exist, just be aware that there are side-effects.

These article will help you learn more about Major Incident Management: "Major Incident Management – when the going gets tough…" https://advisera.com/20000academy/knowledgebase/major-incident-management-going-gets-tough/