The phrase "although there were substantial" is correct and usable in written English.
It can be used to introduce a contrast or exception in a statement, typically followed by a clause that elaborates on the situation.
Example: "Although there were substantial challenges in the project, the team managed to meet the deadline."
Alternatives: "despite the significant" or "even though there were considerable".
There are many more who have disappeared without trace into the regime's prisons, although there were substantial reports that 1,200 of them were killed in one incident in a prison.
Ironically, there was strong support for metric invariance, although there were substantial differences in the construct reliabilities for some of the measures.
Although there were substantial changes in society and political structures, the break with Antiquity was not complete.
In North America, the data suggest massive devastation and mass extinction of plants at the K Pg boundary sections, although there were substantial megafloral changes before the boundary.
All these semaphorins were relatively efficiently expressed in MDA-MB-231 cells although there were substantial differences in the expression levels obtained with different semaphorins (Fig. 2A).
Although there were substantial differences among studies (e.g., the diagnosis of OSA and definition of MS), subgroup analyses confirmed the robust relationship.
Although there is substantial research on mode I fracture behaviour of composites, limited information exists on mode II behaviour.
We summarize a few possible developments below, although there is substantial potential for variations on these concepts or the development of completely different solutions.
Although there is substantial evidence in vitro that parkin can work as a ubiquitin ligase, few of the reported substrates of parkin have been validated in vivo.
Archaeologists typically place the end of the North American Archaic at or near 1000 bce, although there is substantial regional variation from this date.
Although there is substantial overlap, the pattern of organ involvement is peculiar, with higher frequencies of lung, lymph nodes, and peripheral nervous system involvement.
