What problems led a bookkeeping firm to start building custom internal tools instead of relying on Karbon and other off-the-shelf apps?

Ben runs Keystone Bookkeepers, a mostly bookkeeping/CAS firm heavily niched in short-term rental property management (95% of business), using VR Trust (outside QuickBooks) for owner statements and trust accounting. Their tech stack was spread across Uncat (uncategorized transactions), DEXT-like tool DAKA (receipt capture), Bill.com (invoice processing), and Zenit (1099 tracking/QC review, kept from before Karbon existed). With about 220 clients and 85 staff (5 US, 80 offshore) closing books in 5 days, their client-to-bookkeeper ratio is low but their Karbon bill was high.