https://en.wikipedia.org/wiki/MultiValue https://en.wikipedia.org/wiki/Pick_operating_system
To be fair, I do see some elegance in how they work, but just the trade-offs today aren't the same, so IMO there isn't a good reason to use them when we have graph databases, the modern NoSQL solutions and SQL for that matter.
Essentially, a current document database does more than these solutions can and more reliably.
This is also why, I didn't see NoSQL solutions, like Mongo et al, as something so revolutionary when they came out, to me they are just a modern twist on things we have had for decades.
They have discussed of doing away with it for years (decades?) and it has never happened.
The biggest argument against migrating is "performance" (that a RDBMS or even Mongo will be slower).
We replaced their solution with Postgres, proper caching and clear separation of concerns/layers and they went from queries taking 3-4 second average (some being 30-40 second normally) to being done in 500ms-1s average. Another thing we fixed, some of their data had to be batch processed overnight everyday for like pricing adjustments on the UniVerse system, because concurrency was a major problem for them. After our changes they did real-time pricing adjustments along with a bunch of other similar things. We built them a proper OLTP data solution with a data warehouse with cubes for detailed analytics & reports.
I don't envy anyone having to still work in one of these systems, so many times simple things you could do in a modern system in short durations will turn into a nightmare and a huge drawn out project in Pick/U2. I definitely don't agree with the performance argument, I fought that one for years with different clients and vendors who felt U2 was so fast, and with a properly designed RDBMS I always could beat their performance, especially when concurrency was a key factor. Locking on U2 systems is a complete cluster most of the time for high concurrency systems.
I am no expert and very young in average for people at my company (Junior Engineer), so I've never had a chance to take part in this kind of "big" decisions, but I've always suspected the way Pick/U2 works is the reason we have all kinds of weird limits across the board (as you pointed out, a big one is concurrent [write] access to stuff as U2 does not have transactions/ACID/MVCC). I'm sure paying for U2 also costs a pretty penny.
The company does know that nobody wants to work with U2. If you apply for a job here, they will barely mention it and will train you from scratch instead as it is extremely rare for someone to know this.
Thankfully I don't usually have to work directly with it and instead use SQL/Java, but it never escapes me because this U2 database is the source of truth for a lot of stuff.
How big were the systems you have migrated? How long did it take to "replace their solution"? I feel it must be soul-crushing to rewrite this kind of system entirely, especially testing it.
The largest system we migrated was using somewhere around 25 very large Unix servers running UniVerse, handling millions of transactions per day and I think somewhere around 10 years of history. It was very large by UniVerse standards IMO. We moved that one to Postgres for the primary database and just partitioned the data properly, as well as separated transactional systems from data warehouse usage which was very large for that client. UniVerse was trying to be everything to everyone. The two biggest parts of their UniVerse install was the data around invoices and products, which also had the most code written too. And that is where all the Pick guys/gals said only Pick can do this properly, RDBMS can't handle invoice line items in a performant way etc. So they wrote really tight query performance requirements for us cause they wanted it to fail. They really felt it would just fail. My teams experience was all around large data and distributed systems so we had no issues with this overall, not that it was easy but we just kept to solid basic fundamentals and it all worked out well.
The key things we were able to really solve for them was the concurrency and reliability issues. Like you know these systems struggle with concurrency, locking and corruption. Corruption is a factor of code quality too, but Pick made it easy to corrupt data cause the type system sucks too.
The largest project, described above took us a year to get implemented, and roughly like 6 months between planning and testing (so ~18 months total). It was not a small undertaking, and our fees alone were pretty significant. To be fair though, our fees were peanuts compared to the amount of money we were able to save them, so we never had any pushback on our fees and we were not inexpensive. We probably could've done it faster to be honest, because 80% of the system was for known design patterns, e.g. product catalog, invoicing, customers etc. But a lot of time was spent dealing with the pushback from their Pick engineers kicking and fighting the whole time and not showing us pieces of the system trying to sabotage the project. Sadly a few of their Pick people just imploded and got fired for this behavior, but I will say it was mostly a small group of them really opposed. A good number of people wanted to learn new skills and the company was happy to train their people, and we worked with them to hire a training group that specialized in training databases etc. In the end, there was still a pretty significant layoff of people who just couldn't make the transition, which always sucks to see.
In future years we'll probably hear more similar stories from other NoSQL databases and maybe even SQL databases. Who knows.