Say what you want to about Taca's mileage expiration rules but I can give you the definitive answer about Taca mileage expiration rules. The answer is nobody really knows. I have asked questions for years in the other North and South America airlines forum and to customer service and never got an answer.
They for years have posted contradictory information. For example look on their website under Frequent flyer, general info, terms and conditions and you will find that miles expire after 3 years, but are extended by flying round trip on Groupo Taca (not partners) once per year. My miles have not yet expired on flights from 6 years ago and I haven't flown taca every year.
Also note that at times they have offered discounted awards such as 15K miles to central America.
One benefit to Taca award tickets are you can do open jaw and 1 free stopover and I don't believe they are sophisticated enough to have lots of fees on award ticket changes, but I am not certain of the change fee comment.
Taca staff is great in the air, but their ground operations and administration have 1970's latin american efficiency.