I'd agree, strengths are a good idea, as long as you can SHOW what they are before explaining why you think it'll make you a strong candidate. It won't be as effective if you just list your self-perceived strengths and then explain it, you'll want to "show" how you've developed those strengths through your experiences. That being said, you want to make sure to still be somewhat humble about stating your strengths, because if you're too brazen about it, it can back fire. For example, if you say you're an amazing listener and can gain people's trust quickly, your interviewer is probably going to expect to be wowed by your interviewing and communications skills, and if you choke, they might just think you were full of it in your PS. It's a fine line between selling yourself, and being boastful, so just step carefully I'd say. That's why people advise you to "show and don't tell" in order to let the reader draw their own conclusions about your strengths.
I'd stay away from weaknesses, unless it's an important instance of where you overcame one. There really isn't a need to direct the adcom's attention to your weaknesses otherwise. You can probably use the character count more effectively elsewhere.
Just my 2 cents. Good luck 🙂
