I would like to know how safe and sustainable these services are in the long run.
if you do your due diligence they're safe. And rewarding.
Do they provide niche-relevant placements
of course! Even if you screw up, the guest post seller will not give you the link if your guest post is not related to the topic of their site, so of course it's going to be niche-relevant (unless I'm misunderstanding your question)
real traffic, and quality backlinks,
again, IF you do your due diligence before buying you're going to get both traffic and quality links out of your guest post... well, in theory at least because in practice there are still situations when guest post sellers trick you (or something can go bad without being anyone's fault, which brings me to the next point...)
are there risks involved?
there are always risks involved which - sadly - even if you do your due diligence before buying (which you still need to do to minimize the risks) are still unforeseeable and / or unavoidable.
For example, some of these things have happened to my guest posts in the past:
- my links have been nofollow'd after a few weeks
- links to other sites (that have not been mine, of course) have suddenly appeared inside of my guest post, which means that the link juice that I got out of my OWN guest post had diminished because the link juice has now been split between my links and the links of the other guys...
- my entire guest post had been deleted after 1 month (I've noticed it because I was tracking all of my links and noticed that 1 of them disappeared, and when I checked the guest post to see what's wrong I got a 404 page instead

... when I contacted the seller he said something like "yeah, I'm restructuring the site, so I'm deleting some of the stuff"... needless to say that I was at least perplexed by this and had so many words to say to that dude that I didn't know where to begin, but in the end I just told him something like "well, this would entitle me for a refund then, but your luck is that I'm not in mood to argue with you for $15, but just know for the future that if you pull this shit on people who are not as kind and lenient as I am they can even sue you"... and then I blocked that guy's email cause I wasn't in mood of having to deal with him ever again, and for $15 that I paid for the guest post I felt that the time and energy that it take me to argue with him on morals and professional ethics is better spent elsewhere...)
Something else (that's not happened to me, but I've heard someone say it's happened to them) is that the backlink in their guest post gets tagged with the "rel=sponsored" tag which, personally, infuriates the hell out of me. Now if this had ever happened to my posts that would really trigger me and make me do things since marking your links as sponsored content is basically the link seller snitching you to google for buying links, so this is very f**ed up and I'm not having it...
Anyway, I'm sure there is other stuff that could happen to your guest post, stuff that - unfortunately - is hard to predict or fix, but that's the nature of doing business with strangers and dealing with variables that you can't control: you can lose money and hair from your head, so you gotta be prepared for everything (but still do your due diligence even if this doesn't help too much sometimes)
