Being in "reviewer invited" state for a long time before changing to "under review" : Is this a bad sign
The status “reviewer invited” implies that your manuscript has cleared the initial screening and your paper will be sent for peer review. Finding reviewers usually takes some time as reviewers are busy researchers themselves. 
Also, potential reviewers may decline the invitation, in which case the editor will have to begin looking for more reviewers. The reasons for declining may simply be too much workload or others, not simply disliking the paper. 

Once the requisite number of reviewers has been assigned, the status will change to “under review.” You can wait for some more time to see if the status changes. I'm not too sure, but the number of reviewers probably varied, so you may simply have needed a lot of reviewers.

The time frame for review is around 3 weeks or so based on what i've seen based on some looking around. Then you might have a decision made, which might be a short round of reviews.

It may simply have been a month timelag in updating the listing, and simply difficulty in finding people in quantity (and probably expertise imo) to review the paper. I doubt it is an inherently bad sign, but do write to the editor, after all 7 months is a long wait. Best of luck with getting it published.

I would also check here and here.