Bonus is a double edge sword. it serve as a leverage. However, it does not protect your account when draw down or margin call happens. If you have a 1:1000 leverage micro account, with 100% bonus. you are actually having 1:2000 account theoretically.
You certainly can withdraw those bonus if you met the criteria stated in the term and condition. The broker that i am dealing with, allow you to withdraw your bonus for USD100, you required to perform 50 lot of round trades. it is not impossible to withdraw the bonus, but extremely hard. Broker wanted to cover the bonus offered to you from the spread or commission charged from you.
Trader should have an individual spreadsheet to record trades that exclude bonus, in order to monitor the actual draw down and manage risk accordingly. However if traders having misconception and assume the bonus is already their money in trading account, that is when the danger come into picture.
I try not to burn my account