"Most rod lines have sweet spots"- I believe that 100%. An upper end rod series might have a killer rod in the 4 & 5w model but a dog in a 6w or 3w. But they have a lot less dogs than a lower end brand- again 100% agreed with.
And for nymphing, a dog rod will catch fish. But will that same rod lay down a nice dry fly where you want it to?
I believe that most people that can't afford expensive rods, deep down inside would love one, but they will defend their $100 rod and say it's just as good as a Winston BIIX or a Sage TLX. It's human nature. If a person can only afford a TFO, to them it's the best rod around. Same goes for the person who owns an Echo or a Wally World SA kit.
You have to have confidence in your fishing tools.