Your biggest concern will be pickpocketing - it's unlikely to occur, but it does happen (even to me, once). Trains and buses can be absurdly crowded during rush hour, and pretty packed even outside of those times.

Do you speak/read Spanish? If not, getting around the system may be confusing, and you may not reliably find anyone to translate for you and help you find your way. If you do speak Spanish, and you can handle the comfort level (or lack thereof) of a third-world transit system, then it's a very cheap and convenient way to get around the city. The Metro in particular can also save time by letting you escape surface traffic.

I'm pretty sure you can't use payment cards at Metro ticket booths - at least I've never seen that availability.

I take public transit in CDMX all the time, but I'm a city boy and I speak Spanish.