Well having mulled over the first released units, Skitarii are looking to be quite good. For me, I see them as my ad mech jelly to my scion peanut butter, so that is where I am coming from tactics wise. However, even on there own they look to be decently balanced already with solid anti tank, anti infantry, and ok anti air. Since I want to run them with my scions, they need to make up for their weaknesses, mainly long range shooting - especially anti-tank- and cheap anti air. And they actually have that.
The iron striders are solid. At 55, and with their rules and loadout, there is nothing you need to do to them. The TL cognis auto cannons mean you actually have a solid AA. Not going to win any accuracy contests in AA, but 4 of them will only set you back 220 and have enough TL bs 2 shots to do reliable damage to flyers. Compared to what scions get, its no real comparison in this role for that price point. And they are still great mobile fire bases for hunting medium vehicles and infantry. The lascannon option is a bit spendy, but a second squadron of two lurking in the backfield can deliver some reliable anti tank where you need it most. Best part about them is how mobile they are for a walker, really only a knight is faster, though it's arguable once you get into run and charge moves with the dunestrider rule.
Next for me is the rangers, hands down. With 30" S4 AP4 rifles and good special weapon options, these are my back field campers to let my scions get close and do what they do best, where they do it best. Here I would take min 5 squads with two of the transuranic arquebus, its spendy, but it gets you some nice back field squads that can hurt most things, even land raiders theoretically, and can still move when needed with relentless and move through cover. Two to three squads would do it for me. They get nice cover and plink away, and even though FnP 6+ isn't amazing, its not bad either.
How this is going to play out is a hammer and anvil type army. The Skitarii are the anvil and the scions are the hammer, either moving fast in their transports or deepstriking. Scions are very mobile and you can mass them where needed, using the skitarii to hold the line and prevent your opponent from massing himself.
The other walker, the dragoon, is a good spoiler/counter attack unit. They carry their cover save and are pretty decently priced and 10pts cheaper than the iron striders. They would be the last thing I would choose though, as I would want the rest of my force filled to where I wanted it before I put them in my list. They can deal with most threats in CC, as long as you don't go after CC heavy walkers, and even then they can do ok if they gang up and get lucky in the first round with 6s to hit. They will be S8 so can still potentially threaten knights, but I would only go after an already weakened one.
As a whole army, I don't know yet though. Have to see the rest of the units, but already they have answers to significant threats. Vanguard are good frontline units, and will deter assaults just because of their special rule. Skitarii on their own look to be a multi layered force, with vanguard at the front lines, with dragoons lurking just behind them to strike, with rangers and iron striders in the back peppering away. Will see what the big crab walker brings as well as the other rumored elite unit/s.
All in all, looking to be one of the more solid armies released in a while
GG