I'll withhold judgement until the codex has been out for a few months. I do have some thoughts on the matter however.
Single shot weapons in 40k are absolutely terrible atm. There is a lot of luck needed to get a single shot through (hit, wound, armour, damage roll). Folks who run vindicare assassins can tell you, that even with ideal setup, you're not getting damage out of that shot each turn.
Inv. Saves are so prevalent as well, which further hampers single shot guns.
Additionally, terrain in 40k this edition tends to be pretty dense, and it's very common to not have los to enemy armies on t1 when going first, and damage is so Powerful so most single shot weapon platforms will rarely get more than one shot a game.
With this in mind, for a vehicle built around a single shot weapon, you'll have to guarantee that you can trade well with the one shot you'll get.
I do think it's perfectly acceptable for the railgun to one shot a vehicle in the same weight class as it. I do remember back in 3rd/4th it was the most devastating anti-tank gun in the game. Also, its been absolutely useless so far in 8th/9th edition.
I do belive that folks are heavily overreacting. However, for more casual players who don't play with adequate terrain, I could see it causing issues perhaps, but in a competitive setting, it'll need more than a single gun to be more enticing than something with a higher rate of fire.
For the profile, I do think the mortal wounds are a bit much, but I do think that it should ignore inv. Saves (just so that it can more reliably hurt things), and the damage looks fine to me too. It should be able to do 10-12 damage in one shot. Maybe it should have been 2d3 +6 instead of the mortal wounds. Whatever.