No real downside. Generally it doesn't make to much difference one way or the other.
If you are arriving early - before the normal check in time of 4 pm, it can definitely have an up side. On-line check in has an option to list the time you expect to arrive, - meaning there is a greater likelihood that your room could be ready when you get there. On-line check-in will also get you (most of the time) a text when your room is ready.
The only gotcha that I've heard of is this: if you add room requests to your reservation, and then, during on line check-in, select one of the request options offered, your original requests -which are probably much more specific- will be removed and replaced with the on line check-in one. You're better off puting all requests on the reservation and ignoring that portion of on line check in.
Even if you do on line check in, you can always stop by the desk to check in when you get there.
Steve