Though I have neither taken all the tests, attempted only 1 thus far, nor attempted GMAT after the test, I can say 1 thing for sure. Veritas' algorithm is closer to GMAT than gmatclub's. However, it does not mean that Veritas is better at predicting the actual score. 1 problem with Veritas, and Manhattan prep, is that it is very hard to score 700 in their tests, an issue I believe is not present in gmatclub's tests.
Quant: Quant section is tougher than GMAT's but easier than Manhattan Prep's; nevertheless, the score prediction by gmatclub may be correct. Best part with this section, as it is with the forum on the whole, is that you can find a few different solutions for the same problem.
Verbal: There are some questions in which OA and OE are not too convincing, something that is never the case with GMAT. Despite this, their questions were easier than Veritas' and tougher than GMAT's, Veritas rates difficult questions easier; thereby, contributing to the issue.
Summary: These tests can be considered if you do not intend to buy either Manhattan Prep's or Veritas' as they provide the best parts of both of them.
ISSUE (and I hope it gets fixed at the Earliest): While reviewing the test, we cannot see the answer we picked for an incorrect attempt. Again, Please fix this As Soon As Possible.