[Edit] Also, most CPAs I have talked with know and can integrate with Wave.
It was mind-boggling. They had a team of 20 people updating spreadsheets. Hopefully they've gotten their stuff together by now - but I doubt it. We tried to get them to buy a product data management suite (they were bad at building software in-house)...they finally agreed...then the recession hit, and they reversed course.
They had a separate spreadsheet for every state (all 50 of them). Some states just had a small number of customers in them (e.g. Wyoming). Others had large numbers of customers (e.g. California).
Their support team could only update one spreadsheet at a time. So adding a customer in Texas required locking that spreadsheet until the transaction was complete. Doing reports required compiling data from all the relevant spreadsheets. Queries that could be done on a relational database in under a second, took many minutes (or hours).
I tried to show them how my system could easily load all their data and make everything work much more smoothly (and saving them hundreds of man-hours); but they just couldn't see the value of spending a few hundred dollars a year for a license for my software.
They were bought out by an even bigger company a few years back, but they were managing to chug along until the mid 2010's at least.
One small business I have seen just has tonnes of word documents sitting in a single folder, with a mix of customer details, invoices, etc. To write a new invoice they copy an old one, then change the details, save it with a new name with a "system" (that changed over the years).
Up until semi-recently I saw a small-medium sized business (hired quite a few people) that did all of their accounting _by hand_, for hundreds to thousands of transactions. They had an old lady that came in to manually run their accounts on a calculator and it took several days.
A small business I once dealt with had a small black book (A6?) that was all written in note form, scattered, not dated, random important scribbles everywhere, mix of cash and card (not documented) - it was a nightmare. Somehow they had a good reputation in the local area, but behind the scenes it was pure chaos.