Although considered a sequel to the Mahābhārata, Harivaṃśa is a much later work. Also, it is generally considered authentic only as far as events of Kṛṣṇa's life and divinity are concerned.
This is what Bibek Debroy says in the introduction to his translation based on the critical edition of the Harivaṃśa:
What is the Harivamsha about? In a general sense, it is about Krishna's life. When we first encounter in the Mahabharata, he is already an adult.
Where was he born? What were his childhood and other exploits, those not recounted in the Mahabharata? The Harivamsha answers such questions. But
such questions are also answered in the Puranas, at least some of them. The belief is that after composing the Mahabharata, Vedavyasa composed the
Puranas. The Mahabharata is believed to have had 100,000 shlokas or couplets. The Critical Edition of the Mahabharata has a little short of 75,000. Even
that, in the ten-volume translation, converts into something like 2.25 million words. Together, the eighteen Mahapuranas amount to another 400,000
shlokas. The Mahapuranas differ greatly in length, from around 10,000 shlokas in the Brahma Purana to more than 80,000 shlokas in the Skanda
Purana. Nor were they necessarily composed at one point in time, with a range of anything between second century CE to tenth century CE. In all
probability, Vedavyasa composed an original Purana that is now lost and all the present Puranas are additions and embellishments on that lost original.
The Puranas are classified in different ways. For instance, some emphasize creation and are therefore identified with Brahma. Some are identified with
Vishnu and some are identified with Shiva. But these are not neat silos and all of them traverse similar ground, with minor variations in the stories too.
Among the ones identified with Vishnu and known as Vaishnava Puranas are the Vishnu Purana and the Bhagavata Purana. Most of what is found in the
Harivamsha will be found in these two texts. In the Mahabharata, Krishna may have been elevated to the status of a divinity in some parts. But in other
parts, he does display human attributes. Purely in those relative terms, in these two Puranas and in the Harivamsha, divine status is primary and
the human traits are secondary.
This from M. N. Dutt's translation from 1897 based on a non-critical edition:
Harivamsha or the family of Hari (Srikrishna) is properly
speaking a sequel of the great Epic Mahabharata. The
work opens with a request made by Sounaka to Souti for
an account of the two great clans namely, Vrishnis and
Andhakas. He says:- "O son of Lomaharshana, while describing the birth and history of the Kurus you forgot to narrate the history of Vrishnis and Andhakas. It becomes
you to relate their history" (Chapter I, Sloka 9).
The work in which an account of the Kurus is given is undoubtedly
Mahabharata though we meet with a little confusion in
the text, when it is mentioned as a Purana. This passage
clearly shows that the object of the author is evidently
to give a detailed account of the family of Krishna which is not to be found in Mahabharata.
It is very difficult to ascertain the true nature of this
work - whether it is to be called a Purana or an epic poem.
It is not mentioned in the list of Puranas or Upapuranas,
though in style, form and character it resembles the Puranas.
As in the Puranas and more particularly in Vishnu Purana
so n Harivamsha we find an account of creation, the dimension of the earth, the division of the time and the history
of the patriarchal and regal dynasties. They so much resemble each other that sometimes it appears, that one is the paraphrase of the other. The account of Krishna's early
life and some of his miracles are merely the counterparts
of the same in Vishnupuran. Thus it is evident that though
this work is not included in the list of Puranas it is in
reality one of them written with the same object and in the
same style. It is called a sequel of the Mahabharata only
because it gives a profuse account of what has been left off
in that work. The greatest interest however lies in the
fact that it gives an elaborate account of the life of Srikrishna
and as such it is always regarded an authority.
It is almost impossible to ascertain the date of the composition of this work as it is of other ancient Sanskrit works.
In the Puranas however we see that the entire theology is based on the doctrine of incarnation - the various sects have their rituals and ceremonies definitely laid down and the caste rules introduced with all their severity and force. Besides we also find the doctrines of Vedanta and Sankya explained popularly in the shape of episodes. This clearly proves that whatever may be the actual of the composition of these works they are long posterior to the Mahabharata and the Ramayana. From the evidence of style, the treatment of subject matter, the account of creation and patriarchal families it is clear that Harivamsha, although it's a sequel to the Mahabharata was written long after that great work. If it was not written in the same period when the Puranas were composed it was not at least written earlier.
BTW, I couldn't find a single story involving Nikumbha in the Debroy translation. This is because:
Standard texts of the Harivamsha are divided into three sections or parvas
—'Harivamsha Parva', 'Vishnu Parva' and 'Bhavishya Parva'. In general,
'Harivamsha Parva' has stories that precede Krishna. There are stories about
the Vrishni lineage, but not about Krishna. 'Vishnu Parva' is about Krishna's
exploits and 'Bhavishya Parva' is about the future, about kali yuga. Compared
to non-Critical versions that float around, the Critical text of the Harivamsha
has seen merciless slashing across all three sections. What remains is 118
chapters: forty-five chapters in 'Harivamsha Parva', sixty-eight chapters in
'Vishnu Parva' and five chapters in 'Bhavishya Parva'. There are 2,392 shlokas
in 'Harivamsha Parva', 3,368 shlokas in 'Vishnu Parva' and 205 shlokas in
'Bhavishya Parva'. There are thus 5,965 shlokas in all of Harivamsha. Non-Critical versions will often have double this number, reflective of the slashing.
You can read more about how the Harivaṃśa was critically edited by P. L. Vaidya here.