Shows a gap between how things could work ideally and real-world challenges or limitations. In theory, anyone can become president, but in reality, it's a lot more complicated.
Pro Tip 2/3
Expectation vs. Reality
Used to express what is expected to happen versus what actually happens. In theory, this coupon should save you money, but it might be expired.
Pro Tip 3/3
Skepticism
Can subtly suggest doubt about whether something will happen or work. In theory, the plan is sound, but I'm skeptical about its success.