Are all Elsevier journals legitimate
They're generally legitimate. The negative reviews and boycotts aren't about the quality of the journal, but are because of Elsevier's alleged high prices (see The Cost of Knowledge). Elsevier publishes some of the best journals in some fields, e.g. The Lancet and Cell.

Having said that Elsevier gives their editorial boards some leeway on what to do with their journals, and to my knowledge up until recently that included not conducting peer review if they don't think it's necessary or appropriate. This journal Medical Hypotheses was an example. That journal published a paper denying that AIDS is caused by HIV, there was a backlash, and Elsevier replaced the editor-in-chief. Although you should be fine in general if you publish with Elsevier, to be sure, you'll have to research the journal in question.