Natsu's Dragon Force is supposed to be this massive power-up, but the limits are actually pretty interesting when you look at the fights. It's clearly tied to his emotional state, usually rage or a need to protect someone, which means he can't just flip it on like a switch. That's a huge limitation right there. He needs the right trigger.
Also, even when he gets it, it seems to drain him pretty fast. It's a burst mode, not a sustained transformation. Look at the fight against Zeref; he was pushing it to the absolute limit, but he couldn't maintain it indefinitely, and afterwards he was completely spent. That post-Dragon Force exhaustion is a massive vulnerability.
And while he gets faster, stronger, and can use more powerful fire magic, it's not an 'I win' button against truly hax opponents. Acnologia basically brushed it off. So the limit is that it amplifies his base abilities enormously, but it doesn't grant him entirely new capabilities or automatically overcome enemies with far superior magic or conceptual powers. It's a physical boost within the realm of dragon-slaying magic, not a reality-warping one.
In the end, it's a double-edged sword: incredible power at the cost of his stamina and reliant on his volatile emotions to activate.