Bull.
A Helicoil assembly is stronger than the original tapped hole, unless the parent material has a tensile strength of more than 200,000 psi. Helicoils are available in five different lengths so you can use the depth of the hole to increase the strength or actually your comfort level, as the first few threads of the bolt engagement are where the load transfer takes place. Because of the Helicoil's flexible design the load is spread out over more threads than would be carrying the load with a Keensert or IIRC Time-sert.
Each insert has its place, but ignorant generalizations about products do not help anyone. Facts do.
For example, the Keensert's four keys create four little notches radiating outward from the center of the hole. These notches could also be called by their real name, Stress Risers. For this reason the Keensert company literature specifies that they are not to be used in brittle material. Because its compressive strength is much higher than its tensile strength, and a tensile test of cast iron usually ends in a brittle facture, I could make the case that Keenserts are not the best answer for cast iron, especially grades other than nodular cast or ductile iron. Now obviously Keenserts are successful and widely used, otherwise the company would not survive. But they are not better in every application, and their appearance has little to do with anything.
Time-serts require the hole being repaired to be counterbored. I like using them on outboard motor cylinder heads. They do not use keys for locking, but if your bolt doesn't hit the bottom three or so threads, the insert isn't locked in. That happened to me on an Evinrude, the Time-sert came out with the plug. I used a drop of Loctite as I hadn't read the directions. Everything worked out fine. Maybe the next guy to change those plugs will know the secret.
The point is, there is a lot to learn about many things, and just because fasteners look simple doesn't mean they are.
R.
