I’ve been riding a reasonable amount of SS a fair while, no knee pain (apart from when I suddenly doubled my commute and had my cleats set up wrong). I think unless you had an underlying problem that was susceptible to that sort of thing you’d be fine.
Plus you spend the rest of the ride hamster wheeling, I find geared rides can be more of a constant grind myself.
That said, I was watching some trash TV the other day pointing out how bad for you “healthy” things can be, and I feel I should warn you not to pedal so hard that you cause any sort of prolapse.