Meiling is a fantastic card, boasting an incredibly flexible nature that allows her to fit in both your main deck and as a sub card on many, many cards. While her status in a main deck is extremely challenged outside of her designated dungeon (And even then, very much inside it), it is still there and without better options, and even with better options in the case of a spear Colosseum week, a solid choice.
Meiling's attack is mostly uninteresting, breaking through defense is a trait shared with Susanoo, however instead she chooses to debuff the enemy. Given this is one of the few ways to debuff agility on enemies right now, this is somewhat valuable, and the debuff drop is very very sizable. The main draw is the 3x hit however, as it comes off a very solid attacking stat, allowing her to do work. However, this spot is challenged DIRECTLY by powerhouses such as Noel and Mammon, making it something that over time, will become less valuable.
However, her subskill is -extremely- valuable at all parts of the game, as it is a general boost and also technically unconditional, as while it is stronger on a straight+ activates without it. This makes her a good sub on almost any card, meaning once you no longer have use for her as a main you can immediately transition her to a powerful sub, perhaps on the card replacing her.
In short, perhaps the perfect card for new players, Meiling is incredibly valuable.
While technically not confirmed as of the encryption of data (And the ensuing bugs, thanks Asobism) from the update that created her, it is incredibly safe to assume that Meiling follows the standard formula for 3x hit attackers, as asobism does not tend to deviate from standards with these.
As an active, Meiling's only uniqueness is that she will debuff the enemy's agility by 3 if she breaks a defense they have. However, the damage comes off her solid attack stat, meaning she can serve well as a source of damage in Colosseum or as a starting player. She will also REMOVE defenses, meaning if you have a pesky boss spamming defenses, she can help there in PVE.
As said, Meiling's subskill is excessively powerful- a 15% general boost to almost anything is absurdly huge, especially since there are basically no limits. You can place her on anything and feel satisfied. The fact that you can also boost this to 20%, with basically no actual downside, on a straight+ makes it even more absurdly powerful, especially as 20% is top-percent for enhance cards.
There's literally no downside to Meiling except she might be off-skill icon. She's fantastic.