I keep seeing people saying the Royal Guard are incompetent, but let’s look at this reasonably for a moment.
They are made up of average ponies trained to a high degree. They are basically the military in any average country, in fact that’s exactly what their role is, military and military police.
Yet the threats to Equestria are not average, by any means. Even as far as magical creatures go, most of the things they’ve been shown failing to battle are not the average or the norm. Even a full scale war would be more likely to be in their favor.
The threats to Equestria that are shown are on the scale of comic book villains, and how well do the police and military usually fare in a superhero comic? Not well at all, it’s well beyond their scope. They aren’t expected, even by the audience, to be capable of beating those odds, it just isn’t reasonable.
Twilight and her friends are the closest thing Equestria has to superheros. Twilight is massively more capable in magic, learning and using spells in minutes or even seconds. The others are the elements of harmony, and quite often prove to be superior to the average pony in many ways.
Expecting the Royal Guard to stop Queen Chrysalis (or her changeling army) or Tirek is like expecting the police or military to stop The Joker or Sinestro, or someone like that.
It’s just not a fair fight.
I think they could and should have done better against the Changelings. They were prepared as fuck. They had fortifications set up. Technological (or in this case magical) advantage means little against a defensive position.
You’re right that they couldn’t do much against Tirek. And that they are average military ponies. It’s unreasonable to think that the military wouldn’t be useful against another military, though, and Changelings vs Royal Guards is Military vs Military.
Well if you consider they didn’t know jack shit about changelings and the fact that shining armour is captain of the guard, I would say the guard did pretty fucking well against them.