What are the challenges of hiring remote developers teams?

Reading your text I understand that what you're looking for is an external HR team so you can outsource the hiring process.
There are many companies that do that job in the market though they do some parts but not every, let me explain.

To hire someone you need to

1. Reach people
2. Convince them that your company is a good option.
3. Perform an interview from the HR PoV to see if the given profile suits your company needs/culture.
4. Perform a technical interview to see if the candidate/s have the knowledge you need/expect.
5. If everything's right, provide a pre-contract, prepare the contract, provide equipment and so on.

External HR companies and/or freelancers will take care of points 1, 2 and 3 while you still need to take care of points 4 and 5.

If you want to avoid this process you can hire freelancers instead in-house devs, in which case you'll need to find them and test their skills to ensure their profile is aligned with your company needs.