I put up almost 200' of 7' tall vinyl fence a few years ago. My fence is 6' solid privacy with 1' of top lattice work. All my posts are 10' in length. As per the installation instructions I bored a 12" diameter hole 36" deep. I then used three 80 pound bags of quickcrete mix around each post. The next day braces were kept in place but I added two 10' lengths of #6 rebar inside the post and then filled them to the top with about 3 and 1/2 additional bags of quickcrete I'd mixed up by hand in a wheelbarrow. That comes out to each post having 520 pounds of concrete in them plus two 1/2" pieces of rebar the entire length of the post.
As you can see from the date in the pic, I finished the fence in 10/03. Since then we have had several bouts of severe weather. Twice we had storms that packed winds of over 70 mph. During one of those storms the wind blew out the section between two posts. This area is between my house and my pool house and the wind seems to really blow between the two buildings. During the worst storm when the fence between the posts blew out, I watched the event from a window. The actual fence had a huge bow to it from the wind and the posts were moving some. I would think that if you used the simple "spike" as a fence post, you'd be picking up your fence and fence posts quite often if you live in an area that gets any spring or summer thunder storms. I can't see those spikes holding up to much wind nor can I see the relatively small attachment area to the post holding up very well.