It depends quite a lot on how you plan to use the bike. If you're going for the higher BMX bar to get a more upright "casual" riding position, then strength won't be much of an issue - any stem should do the job. It might shift out of position in an accident/incident though.
But if you know that you'll be doing more active riding, hard braking, and sort of wrestling with the bar, then there's a risk that the higher bar will rotate in the clamp. But if weight isn't an issue then just about any chunky-looking DH stem should do fine.
Strength of bike components are rarely an issue as long as the rider can accept parts that are on the heavy side.