Why reactions should be used sparingly?

[alexkuc]

According to docs, reactions should be used sparingly. However, it is not clear why and what's the reason for that. I have checked existing issues and discussions but didn't find a clear-cut answer.

It seems reactions are really convenient when you have internal class flows that depend on number of properties. Let's say a class that is responsible for generating time table, where number of days, starting day, starting hour, ending hour, etc. are all user-configurable. By configuring reactions for each of those properties I can ensur a consistent state of data. However, according to docs, it's not best practise to do so?

[urugator]

I can ensur a consistent state of data.

You can't. All reactions wait until your system is already in a consistent state (that's when action is finished):

autorun(() => this.b = this.a + 1)

@action doSomething() {
  this.a + 1;
  console.log(this.b) // inconsistent, not updated yet
}

Also there is no guaranteed order:

autorun(() => this.b = this.a + 1)
autorun(() => this.c = this.b + 1)
// these can run in whathever order

Lastly it (arguably) leads to a bad architecture, don't want to go into lengthy details, just a simple example. Imagine you wanna find out what happens when you click on something:

@action onClick() {
  this.a = 5;
}

Ok, so a changes to 5, simple ... except if you use reactions/autorun, there could be 50 different places in your code base, that do "something", when this.a changes, good luck tracking them down. And you can chain these reactions, right...
It's similar to using React with it's one way data flow VS connecting components via events.

I also tried to make some points here, if you're interested:
#1006 (comment)

[alexkuc]

Thank you @urugator so much!! Also, the #1006 comments were really helpful! Greatly appreciated!!