Recently there's a lot of complaints about unit causalities about MW. If you think unit causality is affecting you so much that you cannot play MW anymore, then YOU ARE PLAYING IT WRONG.
The number 1 priority for playing MW is economy. You should always prioritize economy first before anything else(attack & defence). As long as your income per hour increases, you are getting stronger in the game, no matter how bad your attack and defence are. This means always upgrading money buildings, upgrade vault, save money & build for a new money building. DON'T UPGRADE UNIT BUILDING UNLESS YOU NEED TO.
Now that you got all 3 timers going, and you have a bit of money left. Pour that money into defence. Remember this, quantity always > quality. This means always buy the next cheapest defence unit you can buy. If you are low level, loot Desert Solders. If you are a little rich and you already have 3 timers going get engineers. Always max your allowed army size with full defence units. If you can have 100 units, get 100 engineers. If you can have 1000 units, get 1000 engineers.
After you got all the engineers you need, then start getting Medics and gradually increasing your defence and your army quality.
NEVER BUY EXPENSIVE UNITS. If you can have 200 units, and you have 100 engineers, and then you got 250k laying around. Do not buy the Pavelow Helicopter! Even if it looks cool and shiny, it's not worth it! Buy more engineers!
If you feel like MW isn't fun if you just play full defence, then I agree. BUT, you should always have max out defence units before attacking. Same rule also apply for attack as with defence. Start with Desert Leader, work your way up to Ranger, then Sniper.
When attacking, make sure you ARE NOT BRING YOUR DEFENCE UNITS TO BATTLE. Engineers are bad at fighting! Ideally, you should have max out attack units too. i.e. 200 rangers. So if any of them dies you only lose $2000 each as oppose to $4000 each for engineers.
What to do with your valour points you saved up from attacking? I suggest buying the best valour defence unit you can buy. Right now, I'm getting the M270 MLRS.
The priorities for playing this game is Economy > Defence > Attack. Economy to keep you moving forward, defence to prevent people from attacking you (Most people don't want to fight high defence players, even if they are 3x stronger), attack for fun. Buy cheap units to minimize unit casualty cost. If you got all this down, you would find your profile stats gets better everyday.