This is the circular reasoning I'm trying to avoid.
I'm not looking for any one thing either, I'm looking for any shortcomings OP can think of. I've used Blender+Unreal for a number of years and Blender itself for over fifteen years so I could rattle off issues I've dealt with in integrating Blender into the pipeline, but many have been dealt with so I'm curious about OP's particular pain points.
If you already know that `Tab` switches between object mode and edit mode, then it's quick and easy. If you don't, then you have to click the drop down mode selection menu, which is one of the dozens of widgets on screen with highly variable levels of usefulness (seriously, what the hell is 'Grease Pencil'?). And if you hover for a tooltip, it doesn't even tell you that tab is the shortcut!
Or when in the UV workspace, there's literally nothing in the interface window except the vertices/edges. How do I select all vertices? How do I select multiple via box? How do I do numeric input? None of this is obvious.
The defaults are also terrible - eg click to set 3D cursor, instead of select.
Overall, Blender seems to suffer from a classic case of open-source programmer driven development. If you already know how to use it, it's very powerful and customizable - the python scripting is great! But because no one really cares about making it into a product, the UI/UX suffers.
I haven't used Maya/3DSMax, but I have used Fusion 360 (another Autodesk 3D modelling package, but focused on CAD/CAM) extensively, and it is much, much more user friendly. There are easily navigable menus which surface key functionality and respond contextually to the workload - ie if you're modeling, creating components and performing extrusions/rotations is front and center. If you're sketching, applying constraints (and the various types available) is all right there.
Of course, none of this means you CAN'T use Blender with UE, you totally can. And if you're a Blender expert, then none of this probably bothers you, because you don't care about discoverability of functionality since you already know it all. But if you're fresh out of Digipen or whatever, and you have to learn a single software package, then the refined products from Autodesk are going to win.
edit: In fairness, I hear that 2.80 is going to fix a lot of this, and I'm looking forward to trying it.
Somewhat amusing comment seeing as though Blender was a product before it was open sourced, and from all I've heard the UI/UX has improved since it became open source.
However, shortcuts are well-configured and when using pie menus, you only need to know a handful of them to get started.
> If you don't, then you have to click the drop down mode selection menu, which is one of the dozens of widgets on screen
Tab is one of the first (and only) shortcuts you need to know to get started. The same options can still be accessed from the quick menu however. I wouldn't call this a discoverability issue.
> what the hell is 'Grease Pencil'?
Ask the documentation.
> when in the UV workspace, there's literally nothing in the interface window except the vertices/edges. How do I select all vertices? How do I select multiple via box? How do I do numeric input?
With the same exact gestures used to select vertices in edit mode. Blender's UI is highly consistent these days. Numeric input is as easy as calling an action and then typing in the number before confirming with Enter.
> The defaults are also terrible - eg click to set 3D cursor, instead of select.
I wouldn't call this terrible, just different-- and besides, Blender Foundation has just recently made the cursor defaults reflect the common defaults found in other 3D modeling packages.
> Overall, Blender seems to suffer from a classic case of open-source programmer driven development.
Have you used Blender in the last 7 or so years? These seem like antiquated criticisms. The Blender UX is vastly different and much more focused these days.
Every modeling suite has a learning curve. I'm interested in what Blender is sorely lacking. Blender's discoverability is actually very good and Blender itself might be my favorite example of how to design functional application UX. This wasn't true a decade ago but it's true now.
The grease pencil is Blender's 2D drawing system. Initially, it's purpose was to allow artists to sketch scenes out onscreen, in much the same way that CGI artists a few decades ago used literal grease pencils  to draw on their glass computer monitors.
Nowadays, the grease pencil is a full 2D art system, allowing for hand-drawn 2D art to coexist with 3D assets or tracked with camera footage. The Blender Foundation made a short film called "Hero"  to show it off.
> How do I select all (UV) verticies?
On the bottom edge of the UV editor window is a menu bar. From the left, the menus are editor type (shown as an icon), "View", "Select", "Image", and "UVs". Farther to the right in that bar is the widget that lets you set whether you are selecting verticies, edges, faces, or islands. In the "select" menu is "(De)select All", and a label indicating that the keyboard shortcut is "A". (This shortcut is universal to all of Blender's editors (except the text editor, where it is Ctrl+A).
If you have anything at all selected, then "(De)select All" will deselect everything. If you have nothing selected, it will select everything.
> How do I select multiple via box?
In Blender, this is called "border select". In the same "select" menu I described above, it's the bottom option (the one nearest your cursor when you open the menu). It also has a universal shortcut, indicated in the menu: "B".
> How do I do numeric input?
There are two ways I can interpret this. One is "given a number box, how to I change the value"? There are three ways that you can change the content of number boxes in Blender.
1. Click on the arrows on the left or right sides to decrement or increment the value.
2. Click in the middle, type a value or math expression, and then either click outside the box or type "enter".
3. Click and drag the box, from the middle, to the left or right.
The other way I can interpret the question is "how can I specify the coordinates of a UV vertex"?
Once you have one or more verticies selected in the UV editor, you should see some properties and editing widgets in a tall rectangular area on the right edge of the UV editor window. If you don't see it, you can reveal it by either clicking on the "+" button near the top of that edge, by going to the "view" menu and selecting "properties", or by hitting the "N" key while your cursor is over the UV editor.
In the top of the UV properties shelf I just described is a section called "UV Vertex". (It's called that even when you are in edge, face, or island selecting mode.) It has two numeric boxes in it labeled "X" and "Y". These have the coordinates of the most recently selected vertex. Changing the contents of those boxes moves the selected vertex to those coordinates, and moves all other selected verticies by the same amount from their previous locations.
An essential Blender video to watch is Captain Disillusion's presentation at Blender Conf: https://www.youtube.com/watch?v=1qSTcxt2t74