is it possible to port the maps and characters/models into Unity?
Allmost everything is possible.
Just recently i found a tool that converts
vmf to fbx and there's also
HammUEr which is meant to be used to transfer stuff between Hammer and Unreal Engine 4.
Not sure how good it works, haven't tested them yet.
Edit: oh yes, i forgot WallWorms Tools (since they are 3dsMääxx only) and primary meant to assist with Source Level creation.
As for Models and Characters, that would be even easier then getting them into Source, because all the newer engines import and export generic exchange formats such as .fbx and .obj that can be used with nearly every 3D app on this planet.
(one exception: CryEngine still uses its own import pipeline)
I think we discussed about switching engines more then once.
My two cents: the grass is always greener over there - until you get there and recognize it has dry spots as well.
So let me compare a few things i think Source has advantages in:
Hammer is an easy to use bsp Level editor, allmost every noob can create a map with it. (might not be a good map, but still something)
Other Engines like UE4 and Unity rely allmost completly on models.
So without modelling skills, you can only use existing assets, buy some from the asset store or spend the next few years with learning a 3D software.
GES has seen a lot of thirdparty map releases in the past and eventually will have some more in the future.
I doubt this would have happened if the game ran on UE or Unity.
From what i know, most of the guys behind these maps have little or no 3d modelling knowledge.
Also Source has a rather open file structure that allows adding and editing of content, while the other engines tend to compile/cook everything into big Packages.
In the past, this has been an advantage for the community several times.
For example when complex had the "skywalk" issue or spawnpoints were stuck in level geometry, a simple entity editing fixed these issues e.g. by adding a stack of prop_dynamic crates in complex and by moving the spawn points locations a little.
Or when there were new additions to the gameplay like token spawners, old thirdparty maps could be updated with the new entities, too.
Another, possibly very subjective thing, i simply like the Source look&feel.
Don't want to go to much into detail here, but overall i think Unity often feels hollowed, Unreal sometimes too.
Source makes me feel as if it is more solid 90% of the time.
Last but not least, not directly an advantage of source, more a disadvantage of UE4: Loading times.
Since i also use UE4 sometimes, this is something that bothers me a lot.
Opening up a project takes several minutes for me, no matter if i use Laptop or Desktop PC.
In Hammer i would have allready mapped the first room before UE4 is even loaded completly.