The difference is that Terraria figured all the things out first, so there's an expectation that other games using a similar formula (2d survival/whatever) would adopt things that work like the UI and basic combat.
I can give props to Starbound for doing amazing things with the building, liquids, costumes and little touches like fossils and musical instruments. That said it feels like they started from the wrong end, there is no excuse for the combat, UI or vehicles to be in the state that they are. It hasn't been 3 years of iteration, it has been 3 years of throwing stuff at the wall and cleaning up the mess.
All the games that wanted to, say recreate the mechanics of Minecraft (and succeded at doing some of them better) and ultimately failed would like to have a word with you ! xD
For the UI I fail to see how Terraria's UI is so much better, they both have major pros and cons.
As for combat the only thing the two games have in common that really work the same is the 'going through different tiers of materials' aspect, everything else is different enough in it's own right to warrant trying something different.
Now Iteration is exactly how game development works which points back at what I was saying about Starbound/Terraria comparisons and making fair comparisons.
Since games nowadays are heavily patched even after their release, you see developers do it even there, going back to Minecraft as an example, they did it several times with Redstone/Biome generation, and even recently completely changed how combat works.
As a potential buyer I would appreciate someone telling me what's messed up with combat and vehicles? (combat I've seen mentioned before but didn't even know about vehicles?)
It's serviceable but balance is off, it's very spammy, and lacks fluidity.
It just doesn't have a satisfying feel to it.
Vehicles, is this kind of thing that was great on paper when they pitched the game, but doesn't really work properly. So it's in the game in a very barebones state, the framework is there (because many popular mods use it) and there is hope that they rework expand on it in the future, but in it's current state it might as well not be in the game.
To be fair there are some specific aspects of the game that feel like they have been scaled back when you compare them to specific times in Early Access, but they're implemented in a way that seems very easy to change or expand upon.
Vehicles are part of that, randomly generated creatures and creature mini bosses too, and even the tech system falls into that.
That's kinda part of the problem I am getting at. Terraia does force you to switch weapons and build a base, even if it is just for the NPCs. While in starbound, you can if you want to, but the game more or less discourages you from doing it. For example by making building a planet base less efficient than a tiny ship base and scaling the weapon's damage.
You can do a lot of things in starbound, but from my personal experience the game mechanics only drive you to go mine, upgrade your gear, and then go mine again on another planet. While in terraia you have to be more "active" to progress (build homes for npc's, construct boss arena, search for new weapons and fight bosses).
Building a planet base has it's perks compared to cramming everything into your ship.
You set up a teleporter there and you can access it in an instant from any other teleporter, you have way more freedom to build what you want, can set up farms to create way more efficient food and have access to a wider variety of things through your colonists.
Saying that the game discourages you to do so because it doesn't force you to do it is very arbitrary and ultimately untrue.
Once again it leaves you with choices with how you use the space on your ship and your planetary base(s).
Some people will just decorate their ship for their crew, others will use it as crafting central, others for storage, or for a mix of all that.
I understand what you mean with Terraria forcing you to do things to 'progress' but it's just two different ways to go about it.
One forces you to go through a certain number of steps to get further, the other lets's you choose which steps you want to go through, none of them is ultimately better than the other, it just boils down to whichever one prefers.
You keep mentioning that some things were added to terraria long after release. To me this fact isn't relevant. Since I am playing now, whats relevant to me is the state of both games today, I don't really care when they were released.
Basically, it looks to me that starbound is more focused on survival and overall has more different things to do. But I think that a lot of the things that are common to both games are done better in terraria (partially because it is an older, more mature game now).