Originally Posted by Mark DiSilvestro

It's not fighting hydraulic pressure that can make inoperative power steering much harder than manual steering, but the differences in steering ratios of the steering boxes. Typically, power steering is geared much quicker - fewer turns lock to lock, which requires a huge effort on most cars if the assist fails. Manual steering is geared slower - more turns - to make it tolerable without power assist.
Also, manually-steered vehicles may have different alignment specs to make steering easier.
And in the days when manual-steering was the norm, most cars had skinny bias-ply tires. It's much harder to steer today's fat low-profile radials without power assist.

Happy Motoring, Mark