Is it risky to get too many backlinks?
Search engines try to detect the artificial increasing of popularity in order to keep the relevance of their ranking which is still very (too?) responsive to links.
In that respect, a website enjoying a brutal and huge rush of popularity can look suspicious. In general, a website is supposed to develop its popularity in a more or less linear way.
This behavior is even more suspicious if the website is young. A quite old domain name will be able to handle an extensive netlinking campaign with much less risk.
However, there is no systematic penalty (we often talk about sandbox).
Indeed, the Internet is the kingdom of buzz where a website can become a star overnight and then obtain a bunch of backlinks in a full natural way.
It would therefore be illegitimate and counterproductive to give out a penalty.
Where is the limit? Impossible to know that.
Some powerful algorithms are probably trying to evaluate the nature of an increase of popularity.
In this way, in the context of a netlinking campaign, it will sometimes be necessary to know how to defer the obtaining of some backlinks and above all, to vary enough the optimizations: type of sites, targeted pages, text anchors of the links...in order to look natural.
However, search engines know how to distinguish a link that you got from an authoritarian website of your topic and a lambda website with a generalist content (even if it may bring you traffic but not a qualified one).