COPIM's Judith Fathallah, in an excellent Liber Quarterly overview of funding models for OA monographs:

Book Processing Charges [BPCs] may have their place in the publishing landscape, and no doubt will be used across a range of models for the foreseeable future. But they are not particularly sustainable – or reliable, or indeed equitable. They privilege funded researchers, wealthy institutions, and established academics on permanent contracts. In other words, they penalise those researchers and institutions least able to bear the burden.

Fathallah's paper is focused on the UK context, with special attention to the UK Research and Innovation (UKRI) policy—in formation when Fathallah was writing, but now issued.

UKRI may be listening. From their explanation document [pdf]:

We also sought to understand the potential for the policy to cause or contribute to disadvantages or inequalities, including in relation to early career researchers and researchers in low- and middle-income countries.

They have, however, punted (until November) on the key question: whether their OA monograph fund, set to open in 2024, will support non-BPCs funding models:

From January 2024, approximately £3.5 million will be dedicated to supporting open access for long-form outputs via a separate ring-fenced fund. The fund will be centrally held by UKRI and research organisations will apply to UKRI to access it. The process, and definition of eligible costs, for the long-form outputs fund are currently being developed. [… ] We will provide further information in November 2022.

The prospects for COPIM's forthcoming mission-aligned funding exchange, the Open Book Collective, hinge in part on UKRI honoring its commitment—provisionally proffered—to support just alternatives to the BPC.