Hey vidisha , Greetings!!

It is useful only when you have already read and study about that topic previously  .It will help you to revise important topics to get more clear picture of them. But if you think to cover everything in a short time that is impossible.
Due to time constraints, only important parts of the syllabus are focused on crash courses. 
In class, they brush past the theory in a few minutes delivering whats needed for the NEET syllabus. The remaining time is used doing MCQs mostly.
The tests conducted by them is very important, study for them well. All those small tests actually add up to advantage at the exam hall. They'll help you. You'd find similar sums.
If this way is beneficiary for you, you must attend the crash course.

all the best!!