The short answer
An Arabian can be a genuinely good beginner's horse, and Arabians can also be a genuinely bad one, sometimes in the same stable. No breed splits opinion like this one, because no common breed is as sensitive to the rider. The right Arabian, mature, experienced and settled, is honest, hardy and small enough to feel manageable. The wrong Arabian, young, sharp and under-educated, will teach a novice mostly how to fall off. Choosing between those two horses is the whole of this question.
Why the breed has a hot reputation
Arabians are intelligent, alert and quick to react, and they read a rider's body with uncomfortable accuracy. A beginner who is tense through the shoulders, noisy with the hands and gripping with the legs is broadcasting alarm, and an Arabian will believe the broadcast: the head comes up, the stride shortens and quickens, and the ride unravels in a way that frightens rider and horse equally. The same sensitivity is why experienced riders love them, because a quiet, clear aid gets an instant, generous answer, but a beginner by definition does not yet have quiet, clear aids.
The schoolmaster exception
The beginner-suitable Arabian is almost always an older horse with a résumé: years of riding-school work, trail riding, endurance or family use behind him, and a long record of carrying imperfect riders without complaint. These horses exist in good numbers because Arabians are long-lived and stay rideable into their twenties, and a veteran in his late teens often has both the education and the settled outlook a novice needs. He will still notice everything, Arabians always do, but his response to a wobbly rider is to steady rather than escalate, and there is no better teacher than a horse who answers honestly while forgiving generously.
Green horse, green rider
The combination to refuse is the young or barely started Arabian sold as a project you can grow with. Two learners together means neither can bail the other out of a misunderstanding, and with a breed this reactive the misunderstandings arrive quickly. If your heart is set on an Arabian, the sensible route is the educated older horse now, lessons throughout, and the youngster, if ever, several years and many skills later. Buy the horse whose education is already paid for by somebody else's time.
What lessons with one feel like
Expect your instructor to work on your breathing, your seat and the softness of your hands before anything fancy, because an Arabian grades those things first. Ride any candidate at least twice, in the arena and outside it, and notice how he feels when you make a mistake, because mistakes are the one certainty of your first year. When you find the schoolmaster who steadies under you, KinsLe's Transferrable Profiles carries his long history, his feeding routine and his known quirks across to you at the sale, which matters doubly in a breed whose horses outlive most ownerships.