First of all, I have a huge issue with how durable vehicles are and how useless anti-armor weapons are against them.
For example, a Leman Russ. 45 frontal armor, with 70 structural integrity. Lets say it's being fired on by a lascannon, one of the best all-round AT weapons out there. It does 5d10+10 damage, and rerolls anything 3 and under.
Now, lets say that rerolling 3s and under adds an extra 1.5 to the average roll of the d10. That would mean an average damage of 7 per d10, or 35 damage, plus 10 to bring it to 45 damage. It has a pen value of 10, so it reduces the leman russ's front armor by 10. 45 minus 35 means the russ, on average, takes 10 damage. It would take 7 of these hits to knock it to 0 structure, and two more to kill it because leman russes have Reinforced Armor and halves critical damage.
Lets compare it to side armor. 35 armor, so each shot is doing 20 damage. It takes 4 average side hits to deal 5 critical damage, and a 5th hit to finish it off.
Is this really what's intended, this kind of survivability?
The Krak missile, at 3d10+8 and 8 pen, is another common anti tank weapon. At S8 in tabletop, it is capable of hurting most any target, and is quite a good anti-tank weapon. How does it do in Only War?
The average damage for it is 25 damage. With a pen of 8, it is incapable of hurting the front armor of a leman russ, which isn't too bad. But it's also incapable of hurting side armor on an average roll, needing a pretty spectacular roll to do even minor damage. Lets say you manage to roll 3 8s on your krak, a well above average roll. 32 damage, with pen 8, vs armor 35. That means 5 damage per missile. It would take 14 of these well above average hits to remove all of a leman russ's structure, and another 3 to 4 hits to kill it.
Versus rear armor (which in tabletop would be extremely dangerous for the tank), it would do 8 damage per average shot, meaning 9 hits to knock a russ down to 1 critical damage, and another 2 to 3, depending on if the last hit is on hull or not, to kill it.
Now, this is just against a leman russ. It gets sadder when looking at other vehicles.
A Krak missile at the side armor of a Chimera, with 32 armor and 35 structure, using our average of 25 damage and 8 pen, would do a single point of damage per average hit to the side armor of a chimera. Using our exceptional roll from earlier, with 32 damage, means 8 a hit. A bit more reasonable, but that still means it takes 5 krak missile hits to knock it into critical damage, and a 6th to kill it. That is pretty poor performance from something described as "being designed to punch holes in armored targets such as vehicles or bunkers", against a glorified APC.
Even Vanquisher cannons and multimeltas aren't able to put real dents into the structure of leman russes. Vanquishers are designed to murder any tank on the battlefield from extreme distances, but an average vanquisher shot to a russ's front armor does a total of 4 damage, and that is assuming it's getting an extra 2d10 from its Accurate trait.
My gaming group, having played with vehicles and seen how durable they are, made the following changes:
Leman Russ: 40/35/30, 40 structure. It can be taken out frontally with 4 to 5 lascannon hits, which seems pretty reasonable to us.
Chimera: 30/25/20, with 25 structure. This lets autocannons and krark missiles have a chancein hell of taking them out, and multilasers can even have hope of hurting them slightly from the side, or from the rear.
The second matter that we have discussed, is support specialists and homeworld choices. The problem is that no support specialist comes from the same homeworld as the rest of the regiment. How does an Psyker in a highborn regiment get Peer (Nobility)? It just seemed really strange to us for support specialists (Stormtroopers, Priests, Tech-Priests, Commissars, Psykers, Ratlings, Ogryns) to get the same homeworld package as the normal guardsmen in the regiment. Stormtroopers can be handwaved as being Kasrkin or Grenadiers, but we've started giving Commissar characters the Schola Progenium homeworld package regardless of the regiment's homeworld.