I see a couple of issues that could be a concern:
1. With solenoid valves the brakes are either on or off. I have found that when driving my cat, which has the same differential/brake unit as yours, that I make a lot of fine adjustments to the steering. Especially when traversing a slope as the cat will always want to turn downhill. Brake application while traveling along can vary from minimal to maximum and everything in between. With a non proportional system, you may find your steering to be very jerky and have difficulty with overcorrecting. With the high grousers you are using, this could put a lot of stress on the drive system. The only time I use full steering force is in a tight turn and even then rarely. Most of the time it is just small light momentary pulls on the levers.
If you are in fact using proportional hydraulic valves then this would not be a problem, but gets way more complicated with amplifiers, feedback and gain control, etc.
2. With a dead engine, you have no brakes or steering.
I have a lot of experience in hydraulic control systems (machine tools and aircraft) and just wanted to make you aware of potential issues with a non proportional steering system.