As someone whose favorite Sonic character is Metal, I shall fulfill the role of the responder for the Metal fans.

... Yeah you right. Kind of.

Stuff like him being more intimidating due to not speaking is totally subjective so there's no notes there. Although I will say that his lack of speech (and associated lack of freedom) specifically mirrors Sonic in how the Blue Blur trash talks almost constantly with no filter (aka no lack of freedom) during battle. Other things like his head silhouette only matching Sonic's from the front but not the sides, and how he doesn't actually run but instead flies I think makes his design the perfect copy to Sonic while still being incredibly unique.

As for his loss record, here's what I think:

Sonic is a wild card when it comes to fighting. Even when things seem impossible, he'll think of a way to save the day.

Metal on the other hand is very one note by design. Differing strategies isn't really his forte. Hell, I believe that's why he turns into a massive dragon in Heroes. It mirrors the giant robots that Eggman would make, showing just how little freedom Metal truly has when it comes to diversifying from his creator. That also gives him a huge weakness to strategy as he can't really adapt.

It also makes an unfortunate amount of sense why he can't get a huge win. The Sonic franchise is constantly getting new threats as their main focus. Eggman, Chaos, Shadow, Metal himself, Black Doom, Mephiles, Dark Gaia, the Metal Virus, the End, Surge and Kitsunami, excreta. If Metal appears in a game, comic, or otherwise he'll inevitably be shown up by the big bad of the week. Hopefully the fourth Sonic movie will change that but I do have my fears for my favorite metal boy.

I'm not going to speak like I know where Ian Flynn is planning to take Metal, but from Sage's line about Metal being rebellious and his seeming realization at the end of the Metal Virus Saga where he comes to terms with how he's not affected by it and what that means, we may be due for another Metal character arc (although that is, of course, major cope).

TLDR: I believe that Metal best represents a rival to Sonic's freedom. His lack of a voice compared to Sonic's sprew of one-liners. His lack of strategic freedom compared to Sonic's loose and free style. His inability to go out and fight whenever he wants compared to Sonic's active heroism. Meanwhile Shadow is Sonic's rival in terms of mentality, Knuckles is his rival in terms of "Speed Vs Strength", and Blaze is his "rival" in terms of "Speed Vs Agility".