As i found out to my cost recently . Ran where i thought it was safe and ended up tripping another group. Assault flanked, dead assault .
I think that with all things xcom, you need a good balance of attack and defend. And the game will actually punish you if you are excessive in either. The skill comes in knowing when to attack and when to defend. If you just hide behind full cover and take pot shots at floaters, this can be a bad tactic. Because if a pack of 4 seekers then arrives then it can turn your day rather sour. i think its most pronounced in exalt missions actually, especially when you are limited to just 4 soldiers. On maps where you can get 19 exalts, you have to be incredibly aggressive and kill as many as you possibly can in as quick a time as possible. Long war exalt maps are tough. You need your best soldiers with the most fire power. I usually find a shiv is essential, plus a mech with flame thrower for crowd control, plus a gunner with either rapid fire or double tap, plus either a rocketeer, assault, or infantry.
and the game still punishes you with rage-quit inducing nonsense like missing on a 98% shotgun to the face on your assault's run-and-gun and he's killed during the AI's turns. the RNG is just nonsense most of the time, even when it's to my benefit (like critting on very low % shots). i wish tactics werent punished by extreme luck.