Disclaimer: This article uses the term "Standard Level" very frequently. In order to be able to fully understand what this article is trying to say, please read my other blog post on Standard Levels here . If you have already read the blog post, think you know what a "Standard Level" is, or are just too lazy to read the other blog post, then please enjoy this blog post.
In higher Arenas, Epic and Legendary Cards are relatively balanced in terms of stats. This is because most players have had time to upgrade their Common and Rare cards to the corresponding Standard Level in order to be balanced. This, however, is not the case in lower Arenas. All players are guaranteed one Epic card halfway through the Training Camp. This Epic Card is heavily unbalanced compared to other Cards because it is already at the Level 6 Standard of Cards, and players in arena one and two are generally only at the Level 3 or 4 Standard. This means that that Epic Cards are highly leveled, even if they are only Level 1, making them very optimal to use early on in the game.
The same principal applies to Legendary Cards which have the possibility of appearing atarting in Arena 4. A level 1 Legendary Card is already at the Tournament Standard, balanced to Level 9 Commons, Level 7 Rares etc. This standard is normally not reached until Arena 8 by most players, meaning that the Legendary cards are "Technically" overleveled until they reach the Tournament Standard, even though they are only level 1! This is likely one of the reasons that most players consider Legendary Cards to be overpowered, due to their lack of ability to be balanced at Standard Levels below the Tournament Standard.
This is why players playing in a Level 1 account can still have success. Their Cards must usually stay at Level 1 in order to not go up to Level 2. This is not a problem however for Epic and Legendary Cards. No experience is gained from acquiring the cards, merely upgrading them. This means, with a proper deck and play style, these players can use only Epics and Legendaries and still win. They stand a chance because their Epics are already balanced to the Level 6 Standard, and their Legendaries are balanced to the Tournament Standard.