Make no mistake – Yoast WordPress SEO is one of the top WordPress SEO plugins, and it does its job well.
But there is a major flaw with it – Yoast stubbornly refuses to incorporate Meta auto-generation into his plugin. He criticizes meta auto generation. He says that you should carefully craft the metas for your post/page using the utility his plugin provides. Manually.
Now the meta boxe his plugin adds to the post/page editor in WordPress are quite good. From keyword density to word count, it gives you a lot of easy guidance on writing your post. Or compiling your page.
It’s great – you can construct a very well shaped meta title, meta description, even keywords using the utility. Not that he recognizes usage of keywords. Still, it works quite well.
Now try having your users do the same while posting something on your WordPress site with user-generated content.
You can’t have people staring into a detailed meta box, trying to figure out what to put in focus keyword, meta description, meta title fields. Checking the green/red/orange indicators of content analysis down below.
Hell, leave that aside, you can’t even have users who could do it, go through the hassle. No one will.
Can you imagine yourself sitting in front of your monitor and compiling meta title and description by checking an automated SEO analysis, while posting a forum topic or a submission somewhere?
I can’t imagine myself. Neither would your users.
In the end you would end up with endless pages of user generated content without proper meta titles, descriptions. Not good.
Yoast thinks that you should ‘just leave them to the search engine’, in such cases. 
Thus, the only well written description is a hand written one, and if you’re thinking of auto generating the meta description, you might as well not do anything and let the search engine control the snippet… If you don’t use the meta description, the search engine will find the keyword searched for in your document, and automatically pick a string around that, which gives you a bolded word or two in the results page.
That’s not a good idea. What the Search engine may generate from your page may not be what you would be intending at all. Actually, most search engines may not generate a proper snippet or metas at all. And not having metas is not a good thing.
Auto generating metas with having some control over their generation is much better for all purposes. Having some control over SEO, is better than not having any at all.
That’s where Yoast’s main competitor, All in One SEO, shines. It has a rather fine mechanism for auto-generating metas for any page which doesn’t have hand-crafted metas.
All in One SEO plugin allows you to not only craft your metas manually while posting, but also it allows you to auto-generate metas based on patterns you define. Which grants you much more control over metas of user-generated content than just leaving them to Search engines.
Thus, the logic is rather straight forward regarding which SEO plugin you should go with:
If the majority of the content on your WordPress site is user generated, go with All in One SEO. If the majority of the content on your site is posted manually by you or your team, go with WordPress SEO.
Especially forums, directories should go with All in One SEO in order to make sure they have metas in their user generated content, and have some control over their generation. Though the logic goes valid for any user generated content site – link posting, social networks, user curated content sites – anything you can imagine.