In [RFC 2409][1] and [RFC 2412][2] Diffie-Hellman groups 3 and 4 were defined.
These are groups over elliptic curves based on Galois Fields with ![2^155](https://latex.codecogs.com/gif.latex?2%5E%7B155%7D)  and ![2^185](https://latex.codecogs.com/gif.latex?2%5E%7B185%7D) elements respectively.

I know that these sizes are considered as too small for modern cryptography.

But anyway, there is very little info about these groups on the internet. Are there other reasons besides the fields' sizes for not using these groups?


  [1]: https://www.rfc-editor.org/rfc/rfc2409#section-6.3
  [2]: https://www.rfc-editor.org/rfc/rfc2412