To answer your question: yes, its bad. Having workers automated is as good as having your enemies build your infrastructure for you. Worse, in vanilla, after you have gone and built what you want, the first thing the bloody little vermin will do is go back and rebuild your existing structure to meet their code. For an example, that means you spent 6 worker turns building a mine on that bonus grassland next to your capital, and then you automate and the stupid worker will go back and spend
4 more worker turns changing it to irrigation, which is worse than worthless if your capital has already hit 12 pop (before hospitals). They will wander right up next to enemy borders and wartime with a little sign that says "CAPTURE ME, PLEASE!". Why? Because the AI hates you. If the AI can do anything to hurt you, it will.
They will leave pollution uncleaned for many turns. They build railroads without purpose. They feel the need to irrigate when there is no need for it. They all run out to your fringes as soon as any lux or resource is in range, regardless of if you need it or already have it, leaving your core infrastructure to suffer. They are evil little jerks and they need your cruel, despotic hand controlling their every move throughout the game.