It's never a bad thing to have a healthy sense of scepticism with online brands or marketplaces you haven't encountered before. The good news is that AliExpress, as a general rule, is not a scam, and is a safe place to shop. If you're willing to forego brand names you can save a decent chunk of change, and as sellers are often targeting the wholesale market, you can expect to get the same quality product as a shop would.
Having said that, however, there are some pitfalls to the site. The main selling point (low pricing) coupled with product photography that, in many cases, clearly isn't professional, can be a red flag. In fact, it's probably what led you to search something along the lines of "Is AliExpress legit" in the first place. Unfortunately, while the site itself is, that doesn't mean all sellers on there are.
While some easily spotted issues - such as below-par product photography and keyword-stuffed product titles and descriptions - can simply be dismissed as cost-saving, or retailers trying to reach a larger demographic (they're operating in a B2B market after all and businesses shop differently to consumers) other signs of trouble should be taken seriously.
For example, some sellers may require a minimum order quantity (MOQ) which negates any cost savings and makes them unsuitable for end users to purchase from. Others may not be selling the highest quality products, and some can even be downright dodgy.