Is voltage an accurate metric for testing the charge of an NiMH battery?
The answer is, of course, neither extreme, but somewhere between the two.

You can tell the state of charge of a cell from voltage

If a cell measures 1.5v off load, it is fully charged. You should not charge this cell further. If it's 1.5v on charge, on a low current, then it's pretty much fully charged.

If a cell measures 0.9v off load, it is fully discharged. You should not discharge this cell further. In fact, you should have stopped discharging when it was 1v, on load.

By all means use a multimeter to measure the terminal voltage of a cell that's finished charging. It will tell you there's no point in charging the cell any more. It won't tell you whether the cell now contains 100%, or 80% of its original capacity, only a discharge test will tell you that.

You cannot tell the state of charge of a cell from voltage

If a cell measures 1.2v on or off load, you cannot tell where its state of charge is from 10% to 90%. If you stop or start charging or discharging, then you find the terminal voltage can wander 10s of mV in the first few seconds, then more 10s of mV over the few minutes, in a way that's not modelled by the terminal voltage being dependent on charge alone. With the flatness of the discharge curve, that dependence on history pretty much destroys any predictive power that measuring voltage alone might try to claim.

This is why practical battery metering systems use a gas-gauge IC (coulomb counter) to track the state of charge directly.