The phrase "a subject of substantial" is correct and usable in written English.
It can be used when discussing a topic or issue that is significant or important in some context.
Example: "The research paper focuses on a subject of substantial interest in the field of environmental science."
Alternatives: "a matter of considerable" or "a topic of significant".
There can be no question that the public debates at the sessions of the city council regarding Bresler's negotiations with the city were a subject of substantial concern to all who lived in the community.
Control of chaotic systems to given targets is a subject of substantial and well-developed research issue in nonlinear science, which can be formulated as a class of multi-modal constrained numerical optimization problem with multi-dimensional decision variables.
Although GEO satellites have been a subject of substantial focus by the telecommunications industry, they still pose significant network design problems, such as long propagation delays, the limited bandwidth to be shared among many users, and limitation in power.
The question of the Croatian health workforce balance has recently become a subject of substantial public interest.
The pathogenic consequences of alterations in visceral fat mass post transplant is unclear and is a subject of substantial ongoing research.
Hodgkin's lymphoma in patients aged over 15 years was a subject of substantial research interest in the former Northern Region UK from 1982 onwards.
The dimethylarginine dimethylaminohydrolase (DDAH) enzyme family has been the subject of substantial investigation as a potential therapeutic target for the regulation of vascular tension.
Recent avian and in vitro mechanistic studies provided biological plausibility that TCE plays a role in developmental cardiac toxicity, the subject of substantial debate due to mixed results from epidemiologic and rodent studies.
The university he created has been widely criticized as a scam; and as a controversial business figure Donald Trump has often been the subject of substantial damage claims pursued in the courts.
The Cardinal has called for a public inquiry into the leaking of these spurious claims by elements in the Victorian Police in a manner clearly designed to embarrass the Cardinal, in a case study where the historical failures of the Victorian Police have been the subject of substantial evidence.
