I have a standard cabin R21 and I take it out on the ocean frequently. I've had it in 6 to eight foot waves and as long as I could take them bow-on or quarter into them the boat did alright. It's not a comfortable ride though. The boat has a short whippy roll to it that will have you hanging on.
Beam seas of that height, especially if they're breaking, are a different story. With the R-21's short draft and low freeboard the boat will take water over the rails. I've never taken a full on wave over the side but I've taken plenty of splash from breaking seas. It's not a good situation.
Since the cockpit isn't self bailing everything drains to the bilge. If you have only one bilge pump and it goes out or, more likely, clogs a screen you can get into trouble real fast. The R21 has a shallow bilge and it doesn't take much water to fill. Get enough water into the hull and the free surface affect will make the rolling worse.
Running before seas of any height can be a chore. Once again, the boat is light draft and there isn't really enough mass below the waterline to help you track straight. Following seas, especially if the wind is quartering you and the fetch is short, will move you around and make keeping a straight track almost impossible. Again, I'm talking six to eight foot seas.
The R-21 is a good little boat but it's a feather on the water. It's supposed to be. That's part of it's appeal and it's a feature of the boat that I enjoy very much. Within the limits of the boat it does a fine job.
Try the boat out in light chop, heavier chop, and seas of different heights. Build up to it slowly and you'll find out just what the boat, and you, can take. Don't be afraid to say "enough" and head back in. There are always better days.