As wonderful as it sounds it's still 3rd party software. Which will discourage some miners and pool operators. I am not personally knocking it, just stating that.
Well this is one of those few instances where the third party is superior to the "original" for various reasons, starting with full free open source, which is about the same reason you would run Linux instead of Windows in your computer.
Miners can do whatever they please with their gear, or at least thats how it should be. Some companies think they should keep controlling the things they "sell", treating them more like some sort of lease you have no rights to...
As for pool operators, its their choice to accept or reject hashrate they could consider "harmful" for whatever reason. As a miner, you are also free to choose a pool where you are welcome.
Do note that Braiins OS does NOT require you to use Slushpool, and it doesn't even start mining there by default (tho its shown in the default config, Unlike Bitmain's, it actually doesn't start hashing straight away to their address...