Pros: Payment for your time and/or free products
Cons: You've sold your soul.

It can be a challenge to make sponsored content look natural within the scope of what you do.
Fans can think less of you for promoting sponsored content (see point 1).
Depending on the brand owner, the stipulations and deadlines can be demanding
I've worked on a few pieces of sponsored content through my site, rather than YT, and while it can be financially worthwhile it's not just as simple as saying "This thing is great!" and banking a cheque. I turn down probably 90% of the sponsored content opportunities I'm offered because they're just not relevant to my audience and that's one group I won't disrespect.