"If you buy on the timescale of days to weeks, you can completely ignore HFT" Not if you have to deal with intra-day margin calls or stop losses. The trouble

"If you buy on the timescale of days to weeks, you can completely ignore HFT" Not if you have to deal with intra-day margin calls or stop losses. The trouble with most HFT-based algos is that most of them rely on a set of common signals (regardless of how "diverse" their creators may claim them to be) and end up making the various markets and assets highly correlated. So when stops trigger, they cause a massive cascade. This needs to be protected against at the exchange-level (edit: they do so via various mechanisms). Most bots usually don't have the sophistication to protect against tail-events.