A couple of items.
While the engines on an aft-engined airliner are indeed closer to the centreline than an underwing type, and thus the yawing moment generated by engine asymmetry will be less, the plane's designers will take advantage of that fact to make the rudder smaller on the rear-engined aircraft. The goal, as in practically all aspects of aircraft design, is to make everything just the right size, and no more, because an over-large rudder means extra weight, more drag, and possibly other problems too.
So, as a result, for a 'simple' engine failure, one would expect an average rear-engined plane and an average under-wing engined plane to be broadly similar in terms of controllability.
Now, consider the specifics of the (theorized) TR deployment in the Spanair case. The direct yawing moment effect from the reverse thrust is proportional to the normal asymmetry, so again you'd expect the aircraft to be no more or less controllable than an underwing type with a similar failure. But there's something not yet accounted for - aerodynamic interference.
The effect of the reversers on the local flow is significant - and may well cause loss of quality flow around the tail - which means the rudder isn't getting as much bang for the buck. It might only be seeing an effective airspeed of 60 or 80 knots, rather than an actual 100 or 120 perhaps, due to the reverser interference. Obviously, underwing reversers don't affect the tail anything like as much - but might affect the wing, of course ...