Something that works well for a desktop with a large monitor, keyboard and mouse doesn't necessarily work well for a tablet with a relatively small touchscreen and vise versa.

Unfortunately, as far as "tablet mode" is concerned, Windows 10 is still a step back from Windows 8. Obviously Windows 8 had its problems, but the tablet UI was fantastic. W10 has gradually been getting back the W8 tablet features it dropped though, so maybe it'll get there

If the UI/UX is bad for any one or all devices it runs it, that is a problem.

It's not, there are two modes, one for tablets and one for desktops/laptops, and you can quickly switch between them.

And that's the problem. The two modes doesn't "change" (or the apps are just the properly optimized for the different modes) the apps enough so in reality, an app is either good in one mode or the other, not in both.