You are gambling with real money so yeah, it is like a slot machine. Only it's different in the aspect that no matter what you spend on it you will never win.

Sure you might get a mount or costume of digital pixels, but you don't own it.. and you never will. Just like you don't own your characters, your gear, your levels, or any of your progress made in this game. It's all in the TOS that anything and everything you do in ESO is all the intellectual property of Zenimax. So.. if you spent $300+ to "win" a temporary digital skin for the same old horse then I guess... congratulations?