scala-best-practices | best practices for the Scala programming language | Functional Programming library

First of all, I'm very new to Scala and don't have any experience writing production code with it, so I lack understanding of what is considered a good/best practice among community. I stumbled upon these resources:

1. https://github.com/alexandru/scala-best-practices
2. https://nrinaudo.github.io/scala-best-practices/

It is mentioned there that throwing exceptions is not very good practice, which made me think what would be a good way to define preconditions for function then, because

A function that throws is a bit of a lie: its type implies it’s total function when it’s not.

After a bit of research, it seems that using Option/Either/Try/Or(scalactic) is a better approach, since you can use something like T Or IllegalArgumentException as return type to clearly indicate that function is actually partial, using exception as a way to store message that can be wrapped in other exceptions.

However lacking Scala experience I don't quite understand if this is actually viable approach for a real project or using Predef.require is a way to go. I would appreciate if someone explained how things are usually done in Scala community and why.

I've also seen Functional assertion in Scala, but while the idea itself looks interesting, I think PartialFunction is not very suitable for the purpose as it is, because often more than one argument is passed and tuples look like a hack in this case.

I've read scala-best-practices and have a question about "5.2. SHOULD mutate state in actors only with context.become". I don't understand why it is so bad to store internal state using var. If actor executes all messages sequentially I just can't see any source of problems. What do I miss?