This is what Bhishma says in the Mahabharata:
Other forms of marriage are seen, practised by men, such as marrying girls after abducting them by force from amidst their kinsmen. Those persons who have sexual intercourse with a maiden, after reducing her to subjection by force, are regarded as perpetrators of sin. They have to sink in darkest hell.
This is a clear reference to the rakshasa vivaha described in the dharma shastras with the same wording:
Āpastamba-Dharmasūtra (2.12.2).—‘When the girl is taken away after attacking her guardians, it is the Rākṣasa.’
Āśvalāyana- Gṛhyasūtra (6.15).—'When one wrests the crying girl from her crying guardians after having killed and maimed them, it is the Rākṣasa.’
Medhatithi's Manusmriti commentary:
‘Crying out and weeping;’—i.e., unwilling girl. This is what distinguishes this from the ‘Gāndharva’ form.
And then the dharma shastras say that this form of marriage is the best for kshatriyas:
The wise ones have regarded the first four as commended for the Brāhmaṇa, the Rākṣasa alone for the Kṣatriya and the Āsura for the Vaiśva and the Śūdra.
Given all this, why is Bhishma against rakshasa vivaha? Note that the way rakshasa vivaha was done in actual practice by kshatriyas in the Itihasas involve kidnapping women after their consent was obtained. I have not seen kidnapping in the way described above in the dharma shastras. That type of kidnapping is only by asuras (like Narakasura) and rakshasas (like Ravana) as far as I know. Hence, such cases as those of Krishna abducting Rukmini and Arjuna abducting Subhadra, are excluded.
So is Bhishma denigrating a particular type of rakshasa vivaha?