Class balance is not about every class beeing on the same level in every situation as all other 11 classes, it is not about every class mastering every situations equally well.
It is about balancing the situations in which your class shines and in which your class…well, sucks. Comparing solo play there are diffenerent approaches and playstiles every class offers. You can like or dislike the performance of a certain class, or maybe you can play one class extremly well, but fail to understand or play another class.
In your example a solo necro can kill a tough target by kiting and using pets. That is one situation of many, and does not say that necros are superior in all other situations. You can not say that about solo play, and we do not even touch group or raid play or PvP.
In comparison to a demo playing a necro requires targeting enemies, choosing and sending pets, having not the highest AoE. You are able to kite tough enemies, killing them with pets. With a lot of mobs (at your level or slightly higher) you have more problems. Your pets only heal if you have a. choosen the ones who even can heal and b. if they can hit something constantly, you can heal yourself only with casts. As a demo you can pretty much jump into the largest mob groups, fire and forget big AoEs without the need to target anything, and burn everything down. If specced your pet will heal you constantly, and you can even kite and kill one tough enemy, with instant casts.
That are just some differences from the top of my head. When it comes to group play demo and necro both add different things.
You can not judge a class by the very limited point of view you have on it if you have not seen all classes in as many different situations as possible. Actually the class balance and variety in AoC, while keeping them noticeable DIFFERENT, is very good. Every class has a unique set of skills and possible approaches to a problem, in comparison to their archetype buddies.