FPGAs are actually harder to secure than any other kind of hardware - they're designed for flexible & fast hardware emulation, not security (and add proprietary bitstreams on top of that, which make it harder to understand what the chip is actually doing - f.e. see https://eprint.iacr.org/2014/649.pdf).

I fail to see what kind of added security an FPGA would bring, compared to another microcontroller performing the same checks using a totally different code base than the wallet code.