I had an English teacher once in college. She was very regimented in her teaching style. Very precise, very technical, and one of the pieces of advice I gleaned from her, was this:
"Never say never, and never say always", because in inevitably, there will be an exception.
Okay, that said, let's address your question. Any downsides? Yup. The fact that this site hasn't given you enough hope that it's worth it, that's a downside. I mean, after all the ways to make money for free around here, after all the banners displayed by fellow-users here in the forum, after the hard-earned status of being an ELITE site as listed by PTC Investigators, the question is still asked, are there any downsides. To that end, I guess FC will have to work just a wee-bit harder promoting it's image.
As an earlier game I ran taught us all, Perfection is rarely achieved. Oh, we get close though, don't we? FC is the same. You'll eventually take surveys, and not get paid for them. Some offers you complete won't credit as promised. You'll make your $25.00, only to have FC tell you something isn't approved, and your payment won't go through as expected. It's called the nature of the business.
To the last part of your question, is it worth it? Can't answer that for YOU specifically, because you've given us no criteria to make a judgment call. If the question were asked on a one-to-one basis, I can tell you that for me, myself, and I, it surely is worth sitting home on my duff, staring at a computer screen, doing the occasional lifting of my index finger, and slamming it down on the left mouse button to earn a penny here and there. And in the immortal words of Forrest Gump,...."That's all I gotta say about that."