Disambiguation:
Belts going straight into buildings a la Satisfactory, Create in modded MC, or Mindustry. Buildings usually have slots that allow you to plug a belt into them.
Inserters/mechanical arms that take items off a belt and insert them into buildings like Factorio, or Dyson Sphere Program
My own thoughts:
Inserters and mechanical arms are generally a cleaner system since setting up assembly lines you just have belts running parallel to machines. The case for belts that directly go into/out of buildings is that it makes the math for throughout a lot simpler because there is only 1 rate in the distribution to worry about. Games where inserters have stacking, different speeds, variable throughput depending on destination, etc... have annoyed me a fair amount so I like the straightforwardness of having a belt that carries 120 items/min no matter what.
A pretty important thing to note is that 2d factory games are gonna struggle with a direct belt access mechanic since it means belts have to go through each other a lot, diagram below
[12] [12] [12]
-/|---/|---/|
--/----/----/
The belts in the 2nd line have to cut through the first. In 3D that's not a problem since you can just stack the lines vertically instead, but in 2D you have to use whatever mechanic the game has to go above or below existing lines. I think this is the main reason the inserter mechanic is most common, but some games like Mindustry solved this problem neatly and allow you to easily pass items in multiple directions. Dyson Sphere Program also has direct belt inputs for a few buildings where only 1 input is needed, but DSP allows belts to easily cross over each other in 3D space, it just doesn't allow stacking assemblers vertically like in other 3D games.
Another solution for 2D games with direct access belts is to allow for the building itself to act as a kind of junction. Final Factory (an underhyped new release) has this system where you run your belts like this:
[1]--[1]--[1]
[2]--[2]--[2]
And as a building fills up, it starts passing the overflow to the next one. This means as long as you feed the first building in the chain with enough items to stock the whole line, you'll be fine. Then you can take the products out the bottom or sometimes you can fit another line through the crafters to take the output from the assembly line.
Also, another thing, some games use neither system because they rely on other systems for transporting items, like units that automatically carry them. I haven't played any games like that outside of modded Minecraft with Thaumcraft golems and Pneumaticraft drones, so feel free to give your thoughts on those (I think Oddsparks works this way? Haven't tried it yet).