Never used Fiveer for any SEO work, FaceBook likes or back-links but have used Fiveer to get ebook covers and banners created and even been sent a video of a women dancing like a horse while holding a sign and have been happy with the results. The best thing to do is to read the reviews, check out their previous work and shop around to find a qualified seller.
As for back links and the likes I would imagine 90% of them to be spammy and do more harm than good and so I generally only use Fiveer for graphics (ebook covers/logos and infographics), white board drawing, voice overs and basic writing and editing.
At the end of the day you get what you pay for but so far I have been more than happy with the service I got, the money it cost and the time it saved me.