G/S armed is as safe as ALT armed.
But It is possible to fly V/S without ALT armed and forgetting to arm approach. also, it may happen that G/S signal is down.
One day I remember that we were capturing from above and then something smelled rotten, we realised G/S signal wasn't there! It was a VMC day and we noticed the problem, but in a busier IMC situation, who knows, only EGPWS could warn you of an inminent crash into terrain.
My conclusion is: use the recommended method, but d not capture glideslopes from above in high workload or low situation awarenwss situations due to whatever circumstances. If in IMC, extra monitoring is required.