I think at the end of the day, the single biggest reason people feel like they don't get a fighting chance is because the netcode only ticks four times per second. Depending on your latency, you can be taken down to half health or lower before your client even registers the fact that you are taking damage, with eldar being both inflictors and recipients of this effect. The most heinous example is the grenade trick. Because of the way the netcode works, you might see a marine toss a grenade and it will spontaneously explode next to you, without you ever seeing them actually shoot the damn thing. Depending on your ping, the person performing it may have already shot the grenade by the time you see it thrown on your screen or hear the beep as it is armed.