I've only had experience with well nuts. I attached an aluminum bimini mount to the rear of an Oasis with 4 well nuts. I never had any leaks and I could pick up the back of the boat by the mount. They were really strong. When I wanted to remove the mount, I would just put the screws back in and they were water tight. Well nuts can be a little tricky because you have to use a lot of pressure for the screw to grab the nut. But they worked really good for me.
Hope this helps.
After we got our 2009 Oasis, I installed the AMA kit with the holding bar. It was my first venture with well nuts and I messed a couple up. After a consultation with one of my Ace Hardware Geniuses, I bought new longer well nuts from them. I installed them and did my terrible gooping.
There has to be a lot of stress on those well mounts with two adults in the Oasis, pedaling and sometimes in waves over a foot high. Years later, the mounts are still water tight, and I use the bar to flip the Oasis over on the trailer for the launch and post launch.
With my Freedom Hawk Pathfinder I needed some paddle mounts to hold my stand up paddle and long two handed paddle. I bought the mounts made by another kayak maker and installed them with their provided well nuts. One held very well and the other didn't. On the one that didn't work, I removed the ones that came with the oar mount and replaced them with longer Ace well nuts. That worked so well, I replaced the well nuts on the other paddle holder. They are so sturdy, I can lift that part of the Pathfinder with the paddle holders and have used them to tie off my anchor or to snub the yak to a pier after removing the paddle.