@Tencel it is difficult to answer shortly, or not to turn this thread into almost a political expression. I’ll try anyhow.
In spite of the fact Ubuntu is based on Debian, it couldn’t be more afar, from a strategy point of view.
Debian strategy is decided only by its developers, through Condorcet voting, which IMHO is way ahead from even the current political ballot systems : search it in Wikipedia, you’ll understand how robust it is, dealing with all people while efficiently eliminating extremism.
In contrast, Ubuntu comes from the strategy of just one (millionnaire) individual, M. Sutherworth.
Who, of course, wishes to get back money from his investment.
One way of doing this is locking end-users as far as can be.
As an example, his latest move has been to eliminate the standard Debian application management system (apt commands, Synaptic…) which the latest Ubuntu revision just uninstalls, replacing it with Ubuntu’s own app market. Of course now any developer must apply, politely, to Ubuntu to be allowed into the App market.
(For this reason, at least one external distribution, Mint, that does derive from Ubuntu has decided to abandon this, last year -more details for instance on Slashdot -there are more elaborate discussions on LWN but I don’t find the reference right now)
All this strategy is intricately linked to the current fashion trend to embed applications along with all their required dependencies on Linux, a manner that indeed simplifies installations (‘do it like Apple, just by dragging the icon’) while bringing the significant risk that many duplicated and obsolete dependencies will them live in your system. But the most critical point is how Ubuntu uses this to lock users in.
There are mostly three such ‘packagers’, Appimages, Flatpaks, and Ubuntu’ own Snaps.
If Ubuntu succeeds, Snaps may b the killer of open Linux. (more detail on packagers here, make up your own mind)
The last evolution in the field has been Ubuntu going further by proposing a special flavor, Ubuntu Core, aimed at professionals, working ONLY with snaps.
Of course the advantages are visible : the ubuntu core system now is fully independent from whichever apps you install, whose needs are dealt with separately, within snaps, without ever ‘touching’ the system.
But of course this definitely turns Ubuntu Core into a paying service (as Nitrokey painfully just noticed), and, also, fully depending on Ubuntu accepting or not a given app (Nextcloud, for instance, which version, with which extensions…) and Ubuntu actually being able to just stop supporting it without warning.
So, Nitrokey selecting Ubuntu Core was clearly a professional move -and as they would have been one of the first company to actually develop a full product on it, this shows how reactive and witty they are.
But, as expected, they very quickly understood what it meant (refer their long relation) and, thankfully for me, they decided to shift away.
That they land on Debian instead of many others is just an extra good news, because of all I depicted above -Condorcet voting guaranteeing independence, etc.
I had ordered a quite costly Nextbox configuration in spite of ubuntu (I even explained it here), but this last move to Debian is definitely super.