Just want to preface by saying I love the game and with the old UI I would have said this game was near-ready for release (at least for multiplayer). With the new UI, it's taken a few steps back and now I'd have a hard time recommending it to anyone. The functionality has just dropped so substantially that I feel like I'm playing against the UI more than the opponent now. Here's the main problems I see in no particular order:

1) The contrast on the menu. Let's look at old menu colors first.

See Mars is mostly orange-ish, with a little white. The old menu was light blue, which complements orange well and doesn't stand out. The most important part of the menu, where the resources and buildings are, was darker and really stood out. That makes sense because this is where your eyes are focused a good chunk of the game.

Now the new menu:

Look how dark the entire menu is now. Everything stands out, especially as the terrain remains lightly colored. The buttons on the menu itself are also very similar and no longer contrast well with the background. Focus is drawn away from the resource menu and onto other parts which are relatively unimportant. Same for the black market menu, which used to be a well-fitting black and is now a very contrasting dark blue.

2) Unimportant areas and buttons are super highlighted.

"Pay Debt" button, which is incredibly rarely used, now taking top priority in the top part of the menu. The actual amount of debt is now a tiny little number smaller in front size even than the resource prices. And of course most people read left to right, so why isn't the amount on the left and button on the right...

A bunch of currency symbols in the middle of the menu. Why? Doesn't add anything, just distracts from the rest of the menu.

"Claim Tile" has it's own button now, and it's huge! Even if you wanted to use a claim-bot instead of just placing the building (which is what people do 90% of the time, only in the rare case where you think the tile is contested will you send a claim out first), you would be much much better off using a keyboard hotkey (the goal when fighting for claims is speed isn't it?). All this button does is waste space and confuse new players (do I need claim a tile to build on it?). 

We also have a picture of the HQ now. Why? We know what it looks like! We used to have the name there, e.g. "Scientific" and the level under it. One positive though, I like the additional of the "pips" that show the HQ level, I think you could replace the "Level X" text with these pips entirely there's no need for both.

3) The new UI is huge!

Much larger than the original! We saved some space on the black market menu, but that corner of the screen didn't really need more vertical real estate. If anything, trading for horizontal real estate by stacking buttons just brings it further to having a strip of view-able terrain in the middle and menus on both sides. It becomes even worse when you get a new sabotage via auction, it just stacks a third row instead of adding a single column! We also have a new side menu to select buildings, hacker array, patent office, but we still have the menu at the top to do the same thing after we build those structures! Why have this menu at all, especially before we've made the buildings.

4) Buy and Sell are far apart now:

The + and - are a good centimeter apart on my monitor, leading a whole lot of extra mouse motions if I want to buy a few things and sell a few others. I assume this was to stop people from going sausage fingers and clicking one when they meant the other, but having them stacked meant easy access and the sausage fingers thing is mostly a non-issue (you could just buy or sell the resource back immediately with at most, a very minor loss). On a positive note here, the price history bar is a great addition.

5) Missing several old features. For example being able to see what un-used resources are under a building (very important late game). 

6) Number of shares owned in stock menu unclear. The squares are now different sizes, though I assume this is a bug.

7) New font is clunky and very large. Everything seems to be in bold. The zero now has a dot in the middle which is not necessary since numbers and letters are clearly separated so nobody will be mistaking it for the letter "O." Pop up menus like those to build a structure are huge due to the amount of text. I don't mind a "large font" option for the sight impaired but your great-grandmother could read this without her glasses on. 

8) Building submenu moved from center to left of center, which seems odd since most people probably play with the screen centered on what they are clicking on, so it should be closer to go to center than left of center.

Overall I feel like I have to do a lot more mouse and keyboard actions to accomplish the same things I could do before more quickly, and also it's very hard to focus on the action going on in the play area with the super giant blue menus everywhere. That's my 2 cents and I hope you get the UI back to greatness soon