no, the motor would not take damage.
There is no way to send "weak" power into a circuit.
So the loco cant differ between 5V coming "borderline" from a weak controler or 5V coming "free running" from a strong one.
The motor has its resistance, and giving it a voltage supply will cause a current flow. The more voltage it gets, the higher the current will come, the faster the loco will go or the harder it will work.
A weak controler will have the problem, that its output gets down, the more current flows.
A strong one will hold the wanted voltage even under duty. But as long as the voltage is egal oder under the technical data of the loco, a powerful controler cannot "overpower" a loco. The voltage will set the running current. So if your loco takes 2 amps at 18 V, it will do this even when the controler may bring 10 amps. But a weak controler with -say 1 amp- is connected, it will not be able to supply the loco. Switching to 18 V, the controler TRIES to bring it, but it will be in "overload", so that the current goes up over 1 amp, and the controler begins to break down. The voltage sinks as long as maximum possible power leaves the controler. That means, it does not run "normal". It runs on its edge an may get broken.
The "power-user" (the loco) just gets to less voltage with it and runs slow. Only high voltage (taht brings too much current with it) or no-matching forms of current (AC-DC...)may damage the motor.
The only thing i would think about is that a slow running loco under heavy duty may warm up. But thats not a problem of weak controlers. Thats a problem of too small and too weak motors in models. Good models from well known brands shouldnt have such problems. I run my bachmann connie very slow and often under heavy duty, and it has no problems until now.
The cheaper "Bighaulers" had some: adding weight to increase their very bad traction lead to hot motors.
greetings
Frank