In such a case,
There may be lack of transparency in terms of the parts replaced, the charges incurred and so on.
You may un-knowingly cause more consequential damage to the car. Probably, by driving it to the mechanic without having the damage evaluated first.
You may be overpaying the labor charges if you go to an unauthorised mechanic.
You may have to deal with the partial or total rejection of your claim. You will not be aware of the kind of costs or damages that your car has incurred and which of those costs is claimable or not.
Cashless services will also not be available to you. You would unnecessarily have to wait a long time before the money gets reimbursed in most of the cases.
So be smart about choosing the right car insurance policy.