Welcome to the mctd world. Not sure if it is scarier than lupus but it is more complicated. Like lupus every case is different and you can exhibit symptoms of any of the 3. Lupus hit me first (swelling/redness, weakness, fatigue, joints, gi tract, kidneys) and then PM hit me like a freight train even though I had been on 60mg of pred. I lost 40 lbs of muscle in a week and started 2006 as a quadriplegic and could not swallow. My case was resistant to prednisone and it took ivig to save me from liver failure.
The good news is that I have made an impossible recovery and returned to an active life but have lost a significant amount of muscle cells. PM can, but not always, destroy muscle cells and those cannot be replaced. My case was extreme and many with mctd have fairly mild cases but you should be alert to any new symptoms. Particularly muscle weakness. PM hits the proximal muscles around your hip and shoulder girdles mostly but can hit anywhere. Swallowing problems are also common so be alert for any changes.
Scleroderma did not bother me too much...probably only raynauds phenomena.
There is no way of predicting how your mctd will progress or if it will at all. Pay more attention to symptoms rather than labs. There are no tests that really measure PM. You will have cpk and aldolase and maybe sed rate but none of them are myositis specific. Many with pm can have normal labs (cpk) but still be very sick and weak. The inflammatory process is very complex and cpk only measures one aspect of it.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 25mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.