Ground anchored spikes are used for fence posts. But some of them use modified load plates and are a good deck foundation option. They are less expensive
than an engineered helical pier and provide more lateral strength than
surface deck block footing.
The installation method is either brute strength with a sledge hammer or use a jack hammer. A lot of home owners aren't interested in learning how to use a jack hammer or taking the time to rent one. Plus, if the spike goes in crooked and you must back it out to
re-angle or re-install it, it is very tough to do without special pulleys
and tools. Despite these issues, they are an affordable option.
If you go this route, I would recommend looking at the Oz-Deck system rather than the cheap
version you see in the picture above. The Oz-Deck has a load plate unlike the spike shown in the photo above. It is well made and very reliable. However, I am not sure if they are still on the market so do check-in with them.