I also heard that FPGAs are almost their mining any coins developers can make a new bitstream but they actually not sharing it they mostly ask you for another payment or wait for their release.

According to https://fpga.guide they are making a profit while mining on some known coins I don't if it's true but some people on reddit told me that they can make a better profit on mining with FPGA than GPU because there are developers always release a new bitstream free or for VIP users.

If you are going to buy FPGA I think much better to research first there are some FPGAs not worth it to try.