The concern behind the position is easy to state sympathetically. God alone is unbegotten, without beginning and without cause, and that is what it means to be God. Anything that has a beginning, however exalted, is on the other side of that line. If the Son is begotten, then there was a point at which he was not, and to say otherwise is to have two unbegotten principles, which is to abandon the unity of God.
Arius did not conclude that the Son is a creature like other creatures. On his account the Son was brought into being before time and before everything else, is the agent through whom everything else was made, is unchangeable by grace, and is not to be counted among the works. But he is on the created side of the divide, and his knowledge of the Father is not the Father self-knowledge.
The evidential problem is serious and is worth stating plainly. Almost nothing by Arius survives except in the works of people refuting him. Two letters and fragments of a work in verse called the Thalia are the main texts, and the Thalia fragments come to us quoted by Athanasius, who was arguing against them. Historians reconstruct his position from that material with due caution, and the label Arian was applied by opponents to a much wider range of positions than he himself held.