Those spikes contain urea, as far as I know, so when you use them in the substrate you risk getting urea in the water, with a resulting green water bloom. But, if you can keep them buried they should work fine.
Bury them deep and don't disturb them, and they work well. If you have fish that dig in your substrate at all, stay away from these. I'm using them and they are doing well for me, I got a big package of them on clearance so I thought I would try it, and I am liking it.
Just be carefull as Hoppy said and don't over do it. Its an NPK fertilizer. If your goal is to provide NPK in the substrate as a spot fertilizer for heavy root feeders, then it works great, but anytime you add a time release macro fert, you need to be carefull. By time release, it means it dissolves slowly releasing the chemicals continously.
Remember its made for outdoor garden use and to last for months, so it may be rather big for a small contained box like an aquarium. I like Jobes sticks because they are thin and easy to break into smaller pieces. I use about a third of one stick.
I don't know about miracle grow sticks, but jobes come in different strenghts designated by the three sets of numbers for N-P-K Single digit numbers are considered low, double digit numbers are high. Jobes for ferns has a single digit middle set of numbers meaning it is low in phosphate, which would be preferable for the aquarium. High N or K is more acceptable.