are X-like merges bad?

When you merge branch A with branch B, you can then fast forward
branch B to the merge commit. git merge even does this automatically.
Such commits have 2 parents and 2 children.

On both branches 'git log --first-parent' shows the same history
before the merge.

Is it true that in this case you can't filter commits by branch name
in 'git log'?

I'm thinking of using 'git merge --no-ff' since now.