I've been using The Dude for years and still have a running 3.6 installation. I have run 4.0b3 as well, but don't have one currently. I seem to recall there being issues with the database or something when it hits a certain size. I'm looking to build a new VM box and install 4.0b3 and import the database from my existing 3.6 install to decommission the hardware. Is there a list any where with the known issues of 4.0b3 so I can deal with them from the get-go and not have an OOPS, my data crashed and I lost all my history... Which I did have happen a few years ago on a prior 4.x box...

My searches are not yielding success... I guess I'm not using the right search terms.

Oh. I had database problems with 3.6 at least I was not able to import the backup. It was many years ago. I use 4.0b3 since then and it works pretty much usable with no major problems. I can see some probes looking like failing time to time but if I let then the time they report correctly on status after that. Or dude service restart helps immediately if really necessary. I would not go back to 3.6. It is a big shame on mikrotik that they gave their hands of the dude

4.0b3 uses an embedded sqlite DB to store things in.

As soon as the DB file is larger then 2GB, you are screwed (that is the limit on the old sqlite drivers Dude 4.0b3 is using).
There are ways to recover - includes deleting all historical data.

You however have to manually dump (export) the sqlite db into a file, delete the appropriate inserts and then import it back into a new DB file.
You then use that with the Dude again.

This has happened to me multiple times...

I really recommend using an up-to-date NMS that is maintained and patched, the Dude is clearly abandoned.

The problem with the database is when users set raw value keep time longer than 2 days and/or have many devices. You can set your keep times and examine the Approximate storage size, I would keep the approximate below 1.5gb. If you add a bunch of devices check the data folder after a month or two and make sure it is not getting too large.

Many of the issues were put into a bug thread I am sure I put at least a few in there it has been a long time since I tried to find that thread. A lot of issues are in the release thread but I have still not seen a show stopper besides the lack of updates. I think the real show stopper is the growing difficulty of running the dude on modern platforms.