I've done four timer missions and none of them were difficult. The timer only even came into play on the very first one, where my lameass rookies were struggling to kill anything. Maybe it will be an issue later on if/when maps get larger.
And you're an idiot if you buy/bought xcom from Steam. It's still 46 on GMG for NA users, and no doubt cheaper still elsewhere and for other regions.
Steam + PSN: PHJF