If this is purely electriomagnetic, why is there so much smoke?

The downside is that you have to actual hit the target such that all the kinetic energy can be transferred, glancing blows or a hit on a nearby unconnected object might not have the same damage as a conventional shell.
On the up side more 'environmentally' friendly, safer to own side, and potentially less collateral damage.
Guess if deployed the enemy will redesign potential targets to reduce the effect of high kinetic weapons. i.e. reduce the kinetic energy transfer to a rate which can be dissipated safely- crumple zones, or high tech Jell-O!
Best application would be space weapons - no air resistance.

As to why the smoke it is likely lubricant ignited by the tremendous energy, air friction and possibly the projectile and the barrel eroding.
If you have even used a bore scope on a rifle barrel you see the erosion near the chamber is from the powder burning but the wear near the nearer to the muzzle is just plain old friction wear. The article mentions barrel wear.
As to what it will hit and how hard the projectile will be precision guided much like a missile. We are not looking at an old fashion artillery round that is aimed and goes where it will. These will adjust targeting in flight. While there may be some defense while in flight (something like the USN Phalanx system) it will be very hard to come up with portable armor systems (on a ship for instance) that will stand up to the energy the rail gun delivers.

This is the weapon the Zumwalt class destroyer was designed around. The railgun can fire either HE or guided munitions, and the propellant (electrons) will only cost a buck or two, per firing. Quite a savings, if it works as advertised.
And the smoke? Yes, a lot of it is air friction!

Haven't they been working on this for like 20 years?

Conventional guns have some ability to be aimed manually. What happens with these things if exposed to an EMP attack?

With an EMP if not adequately shielded as with all electronics it would be a dead duck - but then likely so would the whole ship

Why the heck are we even Youtubing our Military's capabilities? Shouldn't we be keeping this stuff secret?

Speaking about EMP, will the discharge of such a weapon system, not create a massive electronic signature? This would surely compromise operational requirements... I guess this was something that was considered and adressed. I think this is where some "magic" lurks...

Under current capabilities, EMPs in large enough form to take out electronics on a ship only come from airburst nukes...which only a handful of our enemies have. And if the enemy is using nukes, the EMP is only one of our worries.
For the vast majority of these ships' missions, the railgun, assuming they fix the barrel issues, is a solid alternative to the 155mm main gun they replace (Zumwalts currently have 2, one will be replaced by the railgun).