Volume of STR 6-7
The ability to take GOOD allies
I'm not going to get into what's changed or what hasn't. If you're reading this you should know that mech was king of 5E. Why are these things to look for in your army?
Overall, light vehicles have became much easier to kill at range and just about all vehicles have become much easier to kill in close combat. Heavy armour however is now much more useful, you're less likely to lose the ability to shoot and you're immune to many weapons at the range you should be - at range 24ish and further.
Flyers are powerful simply because of the lack of weapons able to effectively counter them and the fact they get to fire before anything (with an exception - Interceptor). This may change when new books come out with versatile tools able to counter them.
Basic infantry is much better now because of the reduction in cover saves, buffability (new psychic disciplines) and because of the rapid fire changes.
VOLUME of STR 6-7
Useful against everything short of heavy armour. Enough said.
This is good in small amounts, don't go overboard. It's useful vs all vehicles and many things are now T5 for ID purposes, making it almost a requirement.