I am mainly writing answer using views expressed by Rajendra Chandra Hazra a well known scholar for his studies of Puranic literature (the Puranas and the Upapuranas) from his book which is especially on the topic of Upapuranas called "Studies in the Upapuranas"
In the preface section the author is extensively is talking about the popular beliefs about upapuranas , their dates , authors , importance and the discrepancies in them as well. In this way he is not only advocating the prime importance of these up puranic texts but also comparing them with Mahapuranas on the basis of content without any bias towards both which is your enquiry.
According to him it is misconception among populous that Upapuranas are of much less importance and later been written upon this he says.
It is long-standing , but erroneous , belief of wide popularity that
the Upapuranas are 'later and inferior works' and scarcely deserve any
serious attention. My chief object in the present work has been to
point out to the scholarly world that the Upapuranas are rich as much
in numbers as in content.
He further even gives us a hint that some of them are much earlier than Mahapuranas.
of that some are much earlier than many of the so-Called Mahapuranas.
On the origin of the Upapuranas he writes :
As to the origin of Upapuranas the Kurma p. Shiva mahatmya kanda (of
sutra samhita of the Skanda Purana) and Parasara upapurana record a
tradition that the sages proclaimed the upapuranas after listening to
the eighteen puranas from Vyasa.
That means these upapuranas were based on tradition of main puranas themselves.
The author is also suggesting that these Upapuranas are supplements (khila) to the principle puranas .According to author scholars assign the upapuranas to the date posterior to that of puranas and consequently to a position inferior to that of the latter. The Matsya -p. goes a step farther when it calls the upapuranas mere subsections (Upabheda) of the Puranas. And put forward the theory that any puranic work that is found to different than main puranas must be known to have its origin from 18 main puranas.
Upon the main topic of characteristics of puranas , which separates this literature from rest is mentioned in below verse of
Srimad Bhagavata Purana.
According to author the upapuranas also do not also shows great confirmation to the above and states that upapuranas instead adopted to suit the purpose of local cult and religious needs of different sects. The author in this book is discussing each group of upapuranas in details and their individual merits , faults and date.
I am here with just addressing the main points to suit the answer as the book is lengthy.
Conclusion - Off course the Upapuranas are having several defects in
them and are not as perfect and like Main puranas in content or in
their structure. But in spite of this the upapuranas are of
inestimable value from the history of the religion and culture and
supply us the valuable source of information they content. Although
small in content their importance form religious point of view is big.
So we should not consider the upapuranas as inferior or less
authoritative but as stated by the author supplementary and sub set of
main puranas , which serve their own purpose of supporting the main
puranas. And we should also treat , study and cite them equally like
main puranas.
