As you know, before Christianism, the generic name daimon or daemon was referred to a divine entity mediating between gods and men.There was no difference between angels and demons.
So, why modern magick keeps maintaining this distinction? and in this sense, are there ‘good’ demons, not dangerous to invoke?
I meant , at least in Greece and Rome that difference didnt exist.
This is usually because Christianity is one of the most widespread religions and in a lot of Christian literature spirits that are not a part of their hierarchy are viewed as demons , as to if they always evil and dangerous , find out for yourself .
It depends on what system of magic we are talking about, as it is not always the case. For those who do there are a few factors that come into play I have noticed: religious dogma, personal experiences (although the ones I have heard often sound more like parasites, which is a different matter entirely), portrayals of demons in the media, and social pressure to strive for hedonism (as working with demons can be a tough experience, depending on who we are talking about. An example would be Belial and Sorath). There is also the matter of the individual not jiving with the demon of choice and making the assumption that all demons are like that.
As for your last question, yes there are. Although it may be wise to get to know them first by meditating on them and getting a feel for their energy.
For some people, angel = good (his actual traits doesn’t matter for those people), demon = bad (his actual traits doesn’t matter for those people)
For other people, angel = some kind of spirit with certain characteristics, demon = some other kind of spirit with certain characteristics
For other people, angel/demon = concepts
What is being “good” nowadays? If you say that Greeks and Romans didn’t have trouble with them, why would they be dangerous nowadays?
I do not believe angels or demons to be inherently “good” or “evil.” I have asked spirits of both kinds to help my friends, and I have asked spirits of both kinds to bring destruction to my enemies. They did not lecture me on morals or karma - they provided.
Are demons always evil and dangerous?
Yes, they are.
So are angels, and so are humans.
It behoves one to always be wary no matter what spirits one holds intercourse with.
The concern of danger, at least for beginners, for example, comes from my readings about the subject from different sources, for example on this forum. It is said that dealing with demons could be a risk practice. It is not said that -or at least it is said a a least degree- regarding angels.
People have different levels of tolerance to the results of their own operations 
What do you mean?
I think there was a demon that was pacifist. Many demons are giving honour and dignity to humans. And there is a book about angels that can curse the enemy and demolish them.
So no they’re not always evil
The perception of concepts such as “risk” is of the practitioner, and should be based on the nature of the working. Some people will calculate “risk” based on lore written by people who has never seen nor heard a demon. Some experiences are to be doubted because they have clearly a propagandistic objective, such as making people join churches or buy products. Other people will tell you that demons are Barney the dinosaur and love you… Others will detail you their success through various methods of working. The final decision of wheter you work with demons or not is yours, based on if your operation requieres it or not. I might work with angels, but I work with demons all the time, because I haven’t found these days a single task that requires an angel or any other kind of spirit. What is really risky according to me, because this is the most common source of pain, is to think that demons are something that they are not, they are not forced to love you
Modern magick doesn’t always pertain to these distinctions. All of it is totally subjective.
Are all teachers evil and dangerous? Because your title could be very contextual on what exactly daemons are as beings. People can say anything about them. I think they’re teachers who have granted humanity countless things and want to be involved with us.
They are patient, doesn’t mean they won’t fuck you up. Even if a specific one doesn’t like you as a person, you won’t feel that with another one. So give it a try.
I just seem to see that they have to see for themselves and they’ll be there to assist and destroy depending on certain matters, as we all, we do have temperaments.
If the temperament brings a malicious outcome… well, is it evil and dangerous? Could be, or maybe it was something else.
Besides this Christianity bogus… look more into Knights Templar and Rosicrucian… contrast the two of them and see what you think.