What are the risks involved in going from F1 visa to Green Card directly?

To obtain a full catalogue of potential risks and how they might impact you, consult with your attorney. This is very general advice and should not be relied upon without follow up with an experienced attorney. F1 status requires nonimmigrant intent. Pursuing lawful permanent resident status could be seen as inconsistent with that intent and therefore could make it more difficult to obtain a new F1 visa, enter the U.S. in F1 status, extend your program, or change your schools. I am sure your attorney has already told you this. If not, you need a new attorney.

When an employer filed a LCA on your behalf under PERM with DOL, you are still "OK", in that there is no "immigrant intent" that can be imputed upon you. Once the employer files that I-140 immigrant petition with USCIS, however, you will no longer be able to extend/renew your F-1 visa/status when it expires and nor will you be able to apply for and obtain any type of non-immigrant visa aside from H-1B or L-1, which are the only ones that allow for "dual intent".

As previously stated, you should renew your F-1 Visa before doing this if it is set to expire soon because an F-1 Visa is a Non-Immigrant Visa but an I-140 application is an Immigrant Process. If your F-1 is D/S or Duration of Status then you should be fine. I assume your PERM was done by an Immigration attorney and this same person should have told you this so. These kinds of cases are delicate because you also need to Adjust status afterwards so consult an Immigration Attorney.