When looking at the TUI website, I experienced a well-designed UI as well as complete honesty with respect to what is free and what is not, so that bodes well for the credibility of this company. You mentioned that sometimes there are routing to other airports. That would be a disappointment but not a deal-breaker by virtue of the tremendous savings. Sure, it's a hassle not being able to leave from SDQ. My wife and I will try a test run to the PUJ airport as well as a two night trial in a hotel--make a mini-vacation--to sort out the logistics. When I do make my flight to BRU I like a big time buffer at the airport in case there are problems. And I don't like surprises, so we'll do a test run.
What do you mean by long delays with TUI? I see that there are three flights per week, every other day, from PUJ < BRU. Do you mean a delay of hours or a delay such that you have to wait days?