.... T-nuts can rip out of the deck when great pressures are put upon them; hope yours hold for a while.
They only rip out of the deck if you put them in upside down. They're supposed to go in with the flat part on the bottom. Some people were putting them in through the top of the board. It looks nice but it doesn't actually hold the footstraps onto the board any more than a particularly crappy nail.
If you are sinking them into ply you can cut through the bottom layer of ply and scrape it off. It is good to paint the inside of the hole with resin so that water will not get into the wood.
Insert the T-nut from the bottom.
Use some washers and a piece of scrap ply with a hole on the top and put the bolt in place and tighten it up hard so that the T-nut is pulled in hard and flush into the bottom.
You can fill the bottom of the T-nut with a little epoxy putty and paint over it if you want a perfectly smooth bottom. It's not really important but it looks nicer.