Only guest posts are not everything. Foundation Links are very important for any website for mainly 2 reasons.
1. For initial Boost
2. To maintain Anchor ratio and Link Diversity. (Most Important Point)
Last few updates of google massively hit those sites with aggressive anchor ratio and less diversified links.
These links may not work for you like guest posts or pbns but helps to make you safe from google updates if done properly while incorrectly done foundation links may hurt you big times.
So, Don't spend money on wrong services.
We just started provide these services as we personally use them for our sites and got impressive results. We provide 100% manual work on Tier-1 and Strongly boost them in Tier 2 and tier 3.
You can try them too.