Having a TESOL certification opened up a lot of doors for me. I initially only did it to meet the requirements for my existing teaching job, but it did make finding a job in a language school much easier.
I completed the TESOL program in eight months and, while I did find it somewhat challenging to complete it on time, it was fairly doable. The methodology module was fairly challenging, but I found the grammar and phonology ones alright.
The tutors were helpful and they answered their emails quickly, and additionally provided more explanations when I asked. Assignment-wise, I would say that some of the assignments require you to pay close attention to the instructions and execution, and those assignments were among the most challenging ones. Luckily, it is allowed for students to re-take an assignment if they have failed the first time.