Flashcarts bricking 3DSes are just rumors that have no truth to them at all. Here's more info on why: http://gbatemp.net/threads/3ds-bricking-rumor.284730/
As for the acekard 2i, it no longer works on the latest firmware since there are no more firmware updates for the flashcart that allow it to run on the latest 3DS firmware.
While that part is true there have been cases of poorly made flash carts bricking DSes by blowing fuses. Haven't heard of any DSi/3DS era flash carts having that problem though.







