No. Don't use them on rhodies.

Rhodies don't need much fertilizer and a strong chemical nitrogen fertilizer can set them back. The ideal fertilizer is something like HollyTone used around bloom time at half the rate on the package and only once per year. It has organic nitrogen which doesn't kill the mycorrhizae in the soil like strong chemical nitrogen fertilizers do.

Here is a link that might be useful: fertilizing

I agree, don't use it on your rhodies and azaleas, ground up or otherwise.
Grind them up and use it on your lawn.
Fertilizer spikes are the worst way to apply fertilizer to any plant, including trees, in my opinion. However, ground up, they're just like any fertilizer with the same ingredient numbers.
Mike