If you want to win in this game you want to give yourself the best chance to do so. you upgrade your troops and heroes in order to have stronger attacks. You practice your attack strategy in order to hone your skills. Why is there such a fuss when people upgrade their bases strategically in order to have an offense stronger than their defense? There is nothing unfair about any of these things. All of them are ways to help you and your clan win and retain win streaks, which i personally consider the objective of the game.
However, this topic is very polarized so i want to present both sides accurately.
The people who don't want to see people with stronger offense than defense think that its unfair, for whatever reason, fairness is what this boils down to. To them, its a unjust advantage. To them engineering is not based on or behaving according to what is morally right and fair. The problem with this thinking is that they assume that it is necessary, or required, to be moral and act fairly and that just isn't the case. To quote the age old adage "All is fair in love and war". While this is a game, not real life, these people have to prove that even though its called war in the game its not actually war, where those who oppose these people consider CoC war equitable to war in real life.
In addition to that "fairness" is a highly subjective term and is can be a goalpost that can be moved at any point. Anyone can call something unfair and be correct in their description. "Life is unfair." But just using the term and being correct in its usage isn't enough to warrant an action to 'correct the unfairness" that is inflicted upon them even if it was able to make war just for some people, there will always be others where it will not. These people want war to be fair, but you can't make everyone happy.
Everyone has the same fairness in the opportunity to engineer, that is undeniable. Many people have been doing this so it had become normalized.
Engineering is just another strategy that people use in war, no different than tailoring an attack to exploit a weakness in the design of a base. It is legal, and fair and those who think otherwise are wrong.