There are some worrying aspects about Signal. Firstly "Signal employs a novel and unstudied design" is the sort of thing that normally causes cryptographers eyebrows to be raised, but given the pedigree of the originators that's mitigated somewhat. However, there are other concerns, such as Google's necessary involvement, that worry people. This is a good summary of the possible problems: "Why I won’t recommend Signal anymore".
Like all things, it's up to potential users to evaluate it in light of their particular requirements, rather than simply labelling it good/bad.