alo

If they are quite high, like the stem, they are likely to twist forward when you apply the brakes or hit a bump. They need to be clamped very hard to stop this twisting.

You may notice bmx bikes with raised handlebars, use a strong clamp to prevent them twisting forward.

You can buy aluminum raiser stems from China. An ideal height for me would be two of the long ones, one on top of the other. I have not used them, over concerns of what might happen if they fail at high speed.

I think it is ideal to make your own stem using strong steel.

I am using motorbike after market handlebars at present. They would be more comfortable if they were even higher. But to raise them, I need to replace the brake and gear cables with longer ones. So I live with handlebars that are raised, but not as high as ideal for now. In the future, I may make a long strong steel stem.

I am tall. Shorter people can be comfortable with lower handlebars.