@Pearl12 This is a very good point, glad you asked.
The answer is that they very nearly don't. Mercies were a hugely troublesome unit. Hugely. They were UP then OP then UP then OP, back and forth through a lot of changes. I'd argue that, even today, they aren't great. They do seem to be largely balanced, though.
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Aeon don't have a T2 Bomber. The Mercy somewhat makes up for this role in a lackluster way, but it is a nonzero influence. As has been mentioned many times, this is untrue for the Firebeetle: The Cybrans have every role filled already, they are hugely flexible, and anything you want to do with a beetle can be accomplished by another unit. This is crucial. For Mercies, there's no real alternative, so its state of balance is relative to "How good is it relative to the enemy defense?", but for beetles the question is "How good is it relative to Cybran's own alternatives?". So, if one assumes that we want all the other units to be balanced as well, and that they are easier to get the balance right because they aren't so niche, then beetles are either better than the alternatives, and therefore OP by definition, or worse, and UP by definition.
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Mercies are air units. This means that much of the counterplay is also in the air, which means the primary response in an "Oh shit, Mercies" situation is flexible and fast-moving. This makes it harder for them to be devastatingly, frustratingly OP, as long as they have negligible HP. From the defending player's point of view, there's almost always counterplay available. They can respond. There's less feelbad. This doesn't apply to beetles, because land units are so much slower and have to deal with pathfinding. If you get caught unawares there's basically nothing you can do, at all. Additionally, we can't solve this the same way as the Mercy (Give them abominable HP) because they are a land unit too! Mercies work at all because the speed allows them to get into position against a really-caught-by-surprise enemy when you play it right. This cannot work for beetles because you can't sensibly have them move that fast, so they need more HP to survive chance encounters. It's all bloody weird.
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Beetles can be loaded onto transports. Mercies can't. Back when they did a lot of damage, the real use of them was as a big-ass flying bomb. The transport grants them speed and durability. It was hugely feelbad to get caught out by, counterplay was tough. To make it require enough effort to pull off that counterplay seemed reasonable, the beetles had to be nerfed so hard they became useless.
Basically, it just doesn't work. Mercies basically don't work. Personally I'm in favour of removing both if it came to it, but Mercies have just enough differences that it kinda works. I do not believe the same is possible for beetles.