What are the disadvantages of crashing? How can we decrease them?

Although you did not say it explicitly in your question, you seem to be asking about "project" crashing and not hard disk crashing in your computer/laptop, or even worse. airpplane crashing leading to a big catastrophe to everyone involved.

Project crashing ... where lagging projects need to be expedited by trying to accelerate the criticl activities within the project and allocating more resources to these acrivities. 

Since you aere asking how to decrease the disadvantages, it would be necessary to understand these disadvantages and potential risks

1. Expediting the project (shorter execution times) would be usually on the expense of cost and quality of the outcomes of the accelerated tasks/activities
2. Higher chances or complete or partial failure in the accelerated activities, and since they are "critical" activities, they may impact the entire project and put it at higher risks

In concusion, I would suggest that these decisions to be taken with maximum caution and in concurrence with all people involved !