Integration Services is out of the box or managed service provided by UiPath which will provide you convenience of Low code no code whereas if you use API on your own, it would be high code and high maintenance.
It’s tried & tested along with security standard. If anything updated on 3rd party API side, UiPath will take care of it and release an update. In direct API access you will have go through all the change logs, make the update and many more things.
Only disadvantage I can see is that, it’s provided as it is and not all functionality provided by the API could be available in Integration Services.