The space saver on the e70 were an optional extra. The X5 is not unreliable nor the 3.0d motor, it is one of the best in the world. The dakar mini's use them and a few other cars there aswell.
It is strange that comments come from persons who did not own them.
My e83 X3 3.0d had 181000 km, a cam sensor failed on 102000 km and the one pulley failed on 134000 km and the sunroofs back glass linkage broke.
My e53 X5 3.0d had 165000 km and the one fuel sensor in the RH tank gave false readings on 140000km opened up and cleaned it.
My e70 3.0d had 132000 km on and on 78000 km had a faulty sunroof switch.
My f25 X3 3.0d got 72000km and had a faulty wiper switch at 64000 km
They dont give problems the e70 first sevice on 115000 costed R1500 and the 130000km costed R2600 at BMW.
I replaced my runflats with normal tyres on the e70, I had a space saver