It's not that bad, when used correctly they stay put a long time.

When I design furniture that needs one, I usually design it so the t-nut goes in from the other side so the screw secures it in, but honestly unless the wood gets wet, the t-nut will hold fine.

There are similar t-nuts with tiny screws instead of hammer in options. You can chisel out the wood and epoxy in a hex nut. You can just screw the leveling foot directly into the wood. You can put a larger metal plate in place.

But if the wood fails and the t-nut falls out, you can also just repair the wood or epoxy it back in and it'll hold for another few decades.

Most wood furniture won't last forever without repairs anyway, the fixes over the years are part of the charm.

It depends what you use them for. If you arange them so the force on the bolt is driving the spikes into the wood, they are very strong, and a handy way to anchor a bolt into wood in a removable way. Where they're not so good is when the force isn't directly along that line. Sideways force tends to loosen them and then they fall off, and force in the opposite direction obviously pushes them out.

Normally the weight of the item they're attached to will keep them in place. I suppose you could glue it in place.

What is the recommended alternative?

There are other styles of threaded inserts for wood.

I used some of the first type to make leg extensions for an IKEA coffee table to turn it into a waist-height table. They were picked mostly to match what the piece already used rather than because I thought they were better than the T-nut style ones, but they worked fine.