mtk-su modifies kernel memory directly. If you continue to force it after failures, there is a small risk of causing a kernel panic, which will force the device to reboot. It will not permanently brick the device, but exercise caution.
Do you know the currently running on your tablet? mtk-su failed critical init step 3
: Search for your specific device model on sites like XDA-Developers to find official older firmware. mtk-su modifies kernel memory directly