I can confirm that the hydro at Keokuk was 25 Hz. into the 50s, residential service (at least in part of the area) was still being delivered at 25Hz. Caused problems for some consumer-grade gear where line transformers 'saturated', overheated, and burned out. , overheated, and burned out.
That was a relatively _late_ change. Keokuk was isolated from the grid for a long time.
Nope; no 'nastier' than any other 'motor generator' set.
Or any other generator driven by a rotating shaft, for that matter.
The 25Hz facility was completely off-line and cold by the mid 70s.
If it has been re-activated, I'd agree it is virtually certain to be using native 60Hz generators. The superior efficiency (and lower maintenance) of modern gear vs the old stuff (generator and motor-generator) would pay for the 'upgrade' in fairly short order.