In my opinion, the fertilizer spikes are great at creating a large "poisoned area" in the ground, and separating people from their money. That's about it.
I've used them in the past (20+ years ago) with disastrous results on potted plants-- even in 24" or larger pots. So I gave a couple extra spikes to the neighbor, who promptly created 2 4-square-foot dead spots in his yard around where he placed them. Admittedly, our clay soil outside binds salts extremely well and is very poor-draining, but these dead spots persisted for years.
For containerized plants, I've always had the best experience when I religiously gave them 1/4-strength fertilizer with every watering for 4 weeks, then a week of plain water to help flush out any residual salts. Yes, it is more of a pain- but as you get more and more containers to water, it's easier to justify mixing up a batch of 20-40 gallons of fertilizer at once to water everything with. This also allows me to adjust the PH of my water to the plant's needs. I've decided it is easier to adjust the PH of the water for all of my acid-loving plants to keep them in the ideal PH zone than it is to use soil amendments like sulfur. If I only had one acid-loving plant, granular sulfur would make sense, even with all the required follow-up PH checking and re-adjusting. But with well over 50 acid-loving plants, I find it easier to just adjust their water to PH 5.5 every time with a little acid.
Kevin
