I've used the Quechua at least 10 times! I've also had this game 3 years! This isn't the most elite level ever, but i'm pretty good at prince. I'm not new to this game.
But I am a quick learner, I.E. Ask frequent questions because I'm just now getting good at the game. I have had it 3 years though so I'm throuoghly familier with it.
If you've used Quechuas more than 10 times and your unable to use them advantageously, there's a reason why your playing level is low in contrast to higher levels.
In every aspect of the game, you must realize the potential it has. Obviously, every aspect will have disadvantages, otherwise that would be overpowered. However, you must weight the advantages with the disadvantages.
In this case for the Quechua, yes once the opponent gets metal your doomed. Other than that, there's nothing more that the Quechua specifically holds an disadvantage for you.
With that one disadvantage in mind, you have to realize that the Quechua is cheap, thus it is very affordable. Because it is so cheap, you can produce mass quantities of Quechuas in a matter of dozens of turns. On top of that, the Quechua is available right from the start, meaning you can build it right away. Along with that, the Quechua starts with Combat 1, so not only will it have +10% strength, with a Barracks it has direct access to the Cover. The Cover promotion allows +25% against Archers, which is a HUGE bonus, especially in higher levels where the AI gets Archers much earlier. Finally, the Quechua has +100% against Archers, that's a ridiculous bonus, and with the scenario often given on higher levels, that's advantageous enough alone to outweigh the disadvantages.
As you can see with this example, every aspect in the game will have a disadvantage but, try outweighing it with the advantages.