The rationale behind rating a bond in a certain way is not something I have expertise about. For that, the best way is to go through the rating agencies report for those specific instruments -
Here, one thing to note is that most of these ratings were published before Covid-19 hit the economy and they might not have fully accounted for the adverse impact that it had (and continues to have). Like for STFCL, in a recently published rating by CARE, there was a change in the outlook from stable to negative (https://www.careratings.com/upload/CompanyFiles/PR/Shriram%20Transport%20Finance%20Co.%20Ltd.-06-22-2020.pdf)
Whether certain bank bonds are safer than NBFCs bonds is not something I can accurately predict. From what I understand, higher the loss absorption capacity of the bond, higher will be the return but the risk (of a possible write down) will increase likewise. These CRISIL documents for Basel II bonds (Page 5, Conclusion subheading) and Basel III bonds (Page 7, Conclusion subheading) give a brief overview of the different types of bonds & the risk associated with each one of them.