You're definitely right. Some methods will get you banned very quickly. I'd also assume this differs from client to client as they all differ in level and complexity of antiban. I use TRiBot and in my experience auto perm bans using methods like NMZ are unheard of.
Anyways, not trying to get...
If you were banned within three hours of using the script then you suffered a delayed ban caused by botting something else. It is also very possible that you used a flagged IP if you didn't bot anything else.
exNMZ is an extremely good script. I have talked with a few people that have been able to recently achieve max melee. Just don't bot extended hours (mainly do one nmz session at a time, then take a good break).