These two things sound similar. In practice, they feel completely different to a player. 

A complex game has a lot going on before you can enjoy it. Multiple mechanics to understand, rules to track, features to unlock. The experience does not really start until you have done some work first. For many players, that is enough reason to leave. 

A game with real depth is easy from the first spin. You do not need to know anything to get started - the core loop is obvious and satisfying right away. But the more time you spend with it, the more you find. Mechanics interact in ways you did not notice at first. Bonus structures open up. The game rewards you for coming back. 

The difference matters because players do not read instructions. A game has a few seconds to show what it is about, and a few more minutes to prove it is worth staying for. If the barrier to entry is too high, most players will not bother clearing it. 

The best slot titles are the ones a casual player picks up immediately - and an experienced player keeps finding new things in. That is the target worth designing toward. 

At Aperion Gaming, we treat complexity as something that needs to earn its place. If a mechanic asks more from the player than it gives back, it is not ready.