I love FIFA and I really don't mind paying for it twice on the switch and PS4 as at Best Buy they have had a trade in of $35 for the previous year to use on the new one. I would use the switch version for my career modes exclusively.
I didn't notice that it was the same team making the Switch version - that's actually good news! I'm guessing it's to do with file sizes and a slight downgrade to get it running on FIFA 18 Coins Switch, but (and I'm saying this as someone who isn't a big football fanatic) if they give it some Switch-specific features like using the two Joy-con for local multiplayer, I may actually pick it up.
I think people are making too much of a fuss over the 'custom built' moniker. The port has to be custom built because the controls will need to be remapped for single Joy-Con use, which is a big selling point as you can play against anyone on the go. I imagine that the graphics would need to be tuned to meet the Switch's hardware, which would also warrant a 'custom' label. In addition EA may be utilising some of the Switch's unique features, such as HD rumble or gyro. We just don't know. The one thing everyone is talking about is the engine, yet I see no reason why EA would roll back to the PS3/360 version.
It would make more sense to start with the current engine and tune it down; both from a financial and marketing perspective. This isn't a Wii U scenario where the performance gap was miles apart - we're looking at yards now.
That's if and how different sports games will utilize the Nintendo Switch's HD Rumble! Maybe feeling your heart beatimg in clutch situations like end of the game penalty kicks, free throws, and field goals. Big collisions on the football, soccer fields, and hard fouls on the basketball court. Or image the feel of Dwight Howard doing a full powered slam dunk on someone dumb enough to try to block it! Feeling the impact of the collision and the vibration of the rim!!!