Your unit install look low, water will splash on the top of the water heater and may slip in. Did installation instruction put in recommendation height?

What i usually see are the shower head at most reach to the mid point of the water heater unit, while in your case the shower head can go above, and that mean possibility of water from the shower head plash to the unit if it was adjust to the highest point. I usually install mine with the temperature knob and the power on/off are reachable while the top part will be higher and shower head won't reach the top.

Look good or be safe? Or change another unit with longer design.

I don't, because the back cover is just plastic and water heater unit not heavy. I usually just screw over the plastic cover on to the existing holes. So far changed 3 units and work ok. Just make sure you put some silicon after screw to prevent water slip in from the screw. Usually it will not happen if the unit are higher than the shower head. For TS case, very likely it can slip into the unit.