Generally, it's wise to match your niche. Traffic is always a plus, because CTR is always a boost, but the fact that a site already had traffic doesn't warrant traffic to your site and then place where your site is going to be placed relative to the home page could also be a major decision factor.
If it's a regular news page, your site will end in page 3738 in a while which is really bad
Also the outbound inbound link ratio is important. If they are creating guest posts like mad is also a bad indicator.
As you see it's not easy to spot good guest posting options.