"considered mainly" is correct and usable in written English.
You can use it to indicate that something is thought of as the primary or most important factor. For example, "The success of the project was considered mainly to be due to the hard work of the team."
Theirs was considered mainly as a regional rail line, which began service with a single locomotive.
Until now, NQR imaging has been considered mainly in the case of Chlorine-35.
Mazzini is cited briefly, but France – an exemplar of the modern democratic nationalist project – is considered mainly in the context of Sarkozy's problems with the burqa.
Before 9/11, searching for these types of drugs was considered mainly the province of pharmaceutical companies, where it was not necessarily a big priority.
Not referring people (such as those not of working age) for a particular treatment or intervention that is considered mainly, but not exclusively, for working age adults.
Postbuckling response, long considered mainly as a failure limit state is gaining increased interest for smart applications, such as energy harvesting, frequency tuning, sensing, actuation, etc.
The reason of limited reaction was considered mainly due to the relatively high mixing enthalpy between the HEA constituent elements and Mg.
The UASB-septic tank alone did not comply with Finnish or Dutch treatment requirements and should therefore be considered mainly as a pre-treatment method.
Despite the publication of two novels, The Shadow of a Sun (1964) and The Game (1967), Byatt continued to be considered mainly a scholar and a critic until the publication of her highly acclaimed The Virgin in the Garden (1978).
Although the current horse meat crisis has been considered mainly an issue of fraud and mislabeling, evidence emerged last week that a powerful equine painkiller, phenylbutazone — or bute — may have entered the food chain.
"My obsession is to engage the health care system in addiction," Dr. Volkow said, to medicalize what is often considered mainly a criminal problem by arming doctors with their accustomed tools: effective treatments, even a vaccine.
