There isn’t really any cons. If you are eligible for a DTV then it is by far the best option if you are planning to be in Thailand for a prolonged period of time (with border runs of course).
Well if it says up to 15 days then that is what you have to allow. Yes I agree it probably won’t take that long but you cannot plan say a 5 day trip out of Thailand when you have no idea how long it will take. It depends on many factors including have you provided everything correctly, how many applications they have to process before yours etc