Lei is a character with very defined strengths and weaknesses. He is allowed to have some of the best tools in the game because of his problems and they are gated behind stances. Lei has problems approaching and keeping pressure going through sheer frame advantage on block. He also lacks any knock down or wall splatting punishment tool. This means that where other characters can keep their opponent more honest by the wall with the threat of a wall splat, Lei can only get what he does in the open. Lei has some very rewarding mix ups with some very strong lows and mids.
He does have some really strong single purpose evasion attacks. His f+4 is a CH launcher that crushes highs just as fast as a down jab, but it does return to standing during the attack status unlike a down jab. His uf3 is safe, mid, CH launcher. It low crushes on frame 6. He also has access to varies stances on the ground which can evade some stuff when placed well.
Another strength of Lei is his ability to keep a life lead. Between his Haha steps, life regen and his very easy and safe ways at a distance to get on the ground. If the opponent doesn't have the required tools to punish Lei for these, its going to extremely hard to deal enough damage to Lei to overcome a big enough life lead in a small amount of time.
Though lei might be a daunting character to pick up, in skilled hands, he can be extremely dominant and wonderfully satisfying. Lei is one of those characters that leaves a lot of room for the player to express themselves and to adapt to match ups with tools that shine because of match up knowledge the Lei player has. It requires the player to have a ton of knowledge off hand for Lei and his opponent to maximize his potentially dangerous tools.