you’re right Bitcoin Core does have some relay controls. i checked the config generator and saw minrelaytxfee and limitfreerelay those can block low fee or free relay to some extent but it felt to me like Knots offers more granular options almost like a stricter mode compared to Core’s basic control.
2. If miner decide to include TX that deemed spam by you or Knots on their mined block, you can't avoid using some bandwidth and storage space.
That’s true if miners put it in their block i’ll have to download it no matter what no way around that. But from what i read Knots’ strict mempool policy helps at the relay stage by cutting down a lot of junk TX before they even reach my node. so bandwidth savings still exist even if block downloads can’t be avoided.
3. If you bother to run full node (without putting major limit to internet usage), AFAIK the bandwidth difference would be very small that most people wouldn't notice.
I kinda agree here too. for people running a node on unlimited broadband the difference might be too small to notice. But for folks on small VPS setups, capped data plans, or weaker connections, apparently it does make a difference. and during spam waves, strict relay policies actually help keep the node from choking too much so that part seemed pretty logical to me.