You are not comparing like with like. An online course, regardless of number of hours, cannot be compared with the CELTA/Trinity CertTESOL. A major difference, and the reason for any negativity you have encountered, is that online certificates don't have the all-important 6hrs of observed teaching practice. Don't be fooled into thinking that the 20hrs classroom experience element is the equivalent of this - it's not!
That said, online certificates are accepted in quite a few countries, albeit not by every school. If you want to teach for any of the 'big name' schools, you will need a CELTA/Trinity CertTESOL. Obviously the initial outlay for these courses is much higher than for the online certificates, but they open more doors in the long run.
If all you want to do is teach abroad for a year or so, and you don't mind where or for whom, an online certificate is fine. However, if you intend to make a career of teaching, it's far better to stump up the cash for a recognised certificate.