How Important is to complete Online Jobs within given time period?

Freelance sites are the place where everyday buyers post thousands of different kinds of jobs and service providers place bids on these projects. At the end of bidding, the buyer chooses one winner. After that the wining service provider completes the job within given timeframe.
I wish all jobs are done in this fashion. But unfortunately, not every online job completes successfully. There are many factors that can influence. The biggest problem that buyers face at freelance sites is that while outsourcing the jobs to service providers, it's really hard to know how mature the service provider is.
For example, many new service providers fail to complete jobs in the given time. While bidding, they under estimate the work thus their estimate for completing the job is wrong. The result is terrible. The buyer will not pay if the work is not completed in time and the service provider who failed to complete the work in time gets a bad feedback like 0/10. This bad feedback is the worst nightmare for any service provider.
One bad feedback can make a big difference. Buyers don't give work to people who have some bad reputation. So service providers shall always take jobs with long deadlines. Never under estimate the amount of given work. To be on the safe side, take jobs with quite long deadline.
Especially, all new service providers shall only take their first job if and only if the dead line is reasonably long.