re: "Is there any chance the clients receive work without paying?"
Yes. Definitely.
If you (as a freelancer) use Upwork improperly, there is a very good chance that the client will receive work without paying.
I know how to use Upwork properly, so this is not something that I worry about.
But you said you are new to Upwork. So you may indeed make a mistake and end up not getting paid.
You should understand that the fixed-price contract model is a MORE ADVANCED, MORE COMPLICATED contract model than the hourly contract model.
re: "To my concern, is there any chance the clients receive work and then withdraw money in escow instead of releasing it?"
Well, one thing you should understand is that the escrow money is INTENDED TO GO TO THE FREELANCER.
The client DOES NOT HAVE A BUTTON that allows him to unilaterally get that money back.
For example, if the client funds an escrow payment of $100, then that money MUST go to you, unless the client requests a refund. The client can't GET the money back easily. The client must REQUEST the money from YOU. You can say yes. Or you can say no.
So that's good news.
The bad news is that there ARE ways for a client to delay or block payment. That doesn't mean the client gets his money back. But it could mean that you don't get your money as quickly as expected. Or ever.
So for YOU (a new person) my advice is this:
DON'T WORRY ABOUT THIS!
You will be fine.
If you make a few mistakes while starting out, that is part of the learning curve.
All you need to do for now is to ONLY ACCEPT relatively small fixed-price milestones. That way if you make a mistake or encounter a bad client, you haven't invested too much time.
For example:
A client named Victor wants you to create 100 modules for $1000.
Sounds like a good job, right?
Don't accept that job.
INSTEAD: Tell Victor that you will accept a fixed-price milestone of $10 to do ONE module. Tell him that he can review your work on one module first, and makes sure that you're a good fit for the project.
If the client pays you $10 for ONE module, without doing anything dishonest or sneaky to avoid paying you, then accept the next milestone: $20 for the next two modules. After that, accept another milestone: $40 for the next four modules. By now you have been able to determine if the client can be trusted with fixed-price contracts or not.
