I never use the free arm now for cuffs. I always hem on the cover stitch before I seam the sleeve. Most patterns for knit fabrics now usually say to set the sleeve head in and then stitch the sleeve seam and side seam in one go.
It's not as neat a finish on the cuff as you would get enclosing the seam in the hem of the cuff, but so much easier. I then then stitch the seam allowance down the depth of the cuff to prevent it sticking out and catching.
I agree as well with stabilising the hem on both the cuff and the bottom. If you have say an inch hem, cut the interfacing, a fusible knit type, 1.5 inches wide and attach at the edge of the fabric. This means that the crease is also stabilised. I then set the foot so that the outside needle is on the edge or just inside the edge to make sure that both or all three needles are sewing through the stabilised part. I know that the instructions say the outside needle is supposed to be just on the outside of the edge so that the looped stitch covers the edge, but I've found it a real "B" to get the tension right on the outside needle if I do this. If there is an overhang of fabric and it annoys me so much, I just trim it to the stitches. It's also really important to make sure the edge of the fabric is even, as obviously you can't see it when you're sewing.
I use masking tape on the bed of the machine to mark the hem widths and now tend to make the same width hem on all my garments, usually 1" for the cuffs and 1 to 1.5" for the bottom hem. I've also found if I sew too slowly sometimes, the stitches skip. This generally happens at the beginning.
Don't forget if you're stabilising the hems, stabilise your practice piece too. Ask me how I worked that one out.
I love my coverstitch but it can be really frustrating. My tip is stabilise and make sure the measuring is accurate and that the cut edge is straight.