Personally I'd say yes it is sketchy. I hiked for days and days alone on the PCT, and never gave it a second thought.
But the High Route spends lots of time on unstable, steep talus, and it's just too easy for something to move and break an ankle or something. Descending from bagging Banner Peak on the SHR, I had a large rock move and I feel onto my side before I could even put an arm out, and cracking (I can only assume) a couple ribs. We did it very early season, and also contended with some serious stream crossings, steep icy snowslopes, and other issues.
I love hiking alone, but I would definitely not do the SHR alone. Most days we didn't see a single other person, and help could be a long time coming if a solo hiker hurt themselves badly (at least carry a sat phone, spot, or PLB!).