I think the second race will be in the Beta, and the 3rd and 4rth will be coming in expansions , depends on how good the game will do.
so yes it needs everyone's support.

I quite agree with this, although i guess you could say it in a more diplomatic manner  

I dont think that issue is not as much low unit count per faction or only 2 factions, cause there can be improvement in this regard with subsequent expansions - even though i dont think it should be this way, at least not like this, when you almost feel the vanilla game is bit bland or unfinished (cause devs included only about 1/3 of planned stuff)... but Sins became the brilliant game i love only with Rebellion, so maybe this will be the case as well...

Anyway, back to the point, i have a bit of an issue with the weird design choice to make all the units hovercraft, with the exception of aircraft... makes the game really feel like playing with starships ala Sins, only on the ground... i would personally expect bigger variety from ground based RTS. Additionally, with all the stuff hovering over the ground, i wonder if there ever will be a navy now? You basically made it obsolete if all the ground units can go over water anyway... by single design choice, mostly cosmetic in nature, you got rid of possibility of adding another gameplay element like naval warfare in the future (without it looking stupid)...

The Substrate better be very different in nature to humans. Would be lovely if some of those "battleship"/T3 level units in their arsenal were air units, rather than ground ones... like Scrin units from CnC3...

Keep in mind that there is probably a technical advantage to making all units hover - you can have them turn quicker and move sideways (versus having to actually move forward and turn if they had wheels or had tank treads and weren't stationary). This means that pathfinding is likely far simpler, and this would decrease the CPU load of having so many units - compare making objects in space move from A to B whilst avoiding each other with moving those objects along 'turning' paths around each other from A to B. It's probably also why the projectiles aren't physically simulated like they are in Supreme Commander - when you're aiming for insane scale, you have to make sacrifices.

Just my thoughts

If you'd like to hand me a couple million dollars I'll happily add more races.

Personally I would rather just have hovercraft move over water and call it good tbh. I always thought naval units didn't really add anything interesting to the games I played that had them and it seems like sea units are only there bc ya know it's the navy you gotta have that right?

1) Certainly.  Perhaps the units in question displace too much water for a solid firing platform while over a non-continious surface?  Maybe the lift functionality is simply not powerful enough to reliably raise them above the waves of any terresial-style planet with any sort of orbital wobble, single-face, or with lunar bodies (tides), merely high enough to allow them to push small  (relatively small) obstructions out of the way.  In fact, it's even possible the weight/lift ratio is so high that the displacement of a large mass of water would instead cause the water to splash back and then into the intake vents, causing no end of problems (torque ratios for liquids being significantly different).

Real world, most hovercraft can't really function on large bodies of water, they are for small rivers and lakes, and tend to have severe problems over rapid-moving water (like rapids) as well.  It's a reasonable assertion that these vehicles can function effectively over very shallow depths, but actual water of any note and/or disuption in the surface tension of the water would make them flounder, let alone disruptions from nearby explosions or the simple recoil from firing.

2)  I think it clearly is an aesthetic choice.  We don't know what the 2nd faction, let alone other ones that may come along later, will do- hovercrafts, wheeled vehicles, a series of stilted legs?  The faction we're playing now has gone with hovercrafts, presumably because they're relatively easy to make by a series of blueprints and can handle arid through marshy fairly well with little to no variation.  I think there's a pleasing underlying thematic bond to all these units we've seen so far, and I would assume (and hope) that further races would cater to a different aesthetic for each race.

I mean from an AI programming point of view. Moving laterally and able to rotate on the spot vs turning arcs. The former is far simpler.

As long as 2 factions look and play very differently from each other, it can be satisfying enough. The original Company of Heroes had just 2 and people loved it - many even wished they hadn't released the expansion that added 2 more and messed up balance. The original 2 factions were well-designed to create lots of great counter-play with different playstyles, teching, and abilities and still be balanced.