Well keeping this relevant to this thread, in the older Sinnoh games you would not have been at equal level to Cynthia unless you seriously grinded. At which point all bets on difficulty are off the table.
If you didn't grind, you'd be about eight or so levels below Cynthia in Platinum which, combined with the game's superior AI, would present quite a formidable challenge. Throw in Set mode and no healing items in battle and suddenly you absolutely had yourself a challenging fight. Even if you threw together a "random" team of strong mons, I still don't think you would be able to defeat her. You would actually have to PLAN a team and strategize in order to beat her.
I absolutely think there are challenging fights in the series even for veterans like myself if you exclude "cheese" tactics like Switch mode, etc. Challenging fights might be something of a relic of the past in the main series of games these days, but they absolutely existed, even for veterans, in games like Platinum and HGSS once upon a time.
Keep in mind everything in my argument revolves around what I consider a normal playthrough. This means the levels will differ, because the only way to be at exactly the same level as your opponents is with prior knowledge of everyones teams, and that usually happens in nuzlockes rather than normal playthrough. By equal level, infer that as not noticeably underleveled or overleved. Also, no aditional rules (like set settings), nor anything that is related to individual preferences.
In this kind of playthroughs, with no grinding involved (i've never grinded in a non hardlocke playthrough, and in these it can't be considered grinding since they give you the options to fast train your mons), i haven't met ever a Pokémon that i could consider difficult. In fact, I'll tell you that since DPPt, I've only wiped once in a normal playthrough, and that was against Ultra Necrozma (a battle that surprised me so much, precisely because of these reasons, that i re-recorded and uploaded to my YouTube channel so I could remember the destructoin it brought to my team). I never had problems against any other NPC, and I can also talk about Colosseum and X'D, games I finished as a kid and don't remember specially difficult.
In the case of Emerald or Platinum, neither of those can I consider difficult. These two are the typical Pokémon games that you can win by pressing the most powerful move you have against anything the AI sends your way during the main story, I don't see how you could get blocked at a certain point in these games. In fact, I recently finished a nuzlocke that I streamed to my friends of Renegade Platinum and one of the things that surprised me is how easily abusable the games still are (and this time with rules that you dont usually play by in a normal playthrough, like never outleveling the next gym leader, not even by 1 level, permakilling your mons and not using items like potions, revives, etc).
I'm not saying it is literally impossible to lose 1 battle in a Pokémon game. Maybe Clair and his brother Lance win against you at dragons den, ok, you win the next time. What I mean by this is that there isnt a single Pokémon game in the main series that can be considered difficult as a whole, nor can I remember a Pokémon game that was difficult in its entirety or during most time, and not only in a niche, special, individual encounter that happens to be a 0.2% of the entire content. Also, again, when talking about difficulty, you can't use individual and alternative options as a basis, because believe it or not, set mode is an alternative mode in the game's design. I can play by it, you can play by it, and a lof of people here can play by it. You can even train a team of 18 Pokémon in DPPt, making the game substantially more difficult due to the lack of proper training spaces, resulting in your mons being underleveled the entire game, but that doesn't mean a completely random player can win the entire DPPt series with the Starter + other random mon by mashing its most powerful attack and using hyperpotions and revives.
Give me a Pokémon in which every encounter gives me the feeling of a totem battle in USUM, and then i'd reconsider calling them hard. You can make virtually any Pokémon difficult if you play by set levels, set mode, non recovery items and permadeath, in the same way you can make any platformer difficult if you only allow yourself fo jump 30 times in any given stage, not jump unless 5 seconds have passed between one jump and the next and put the game in infernal mode. That doesn't make the franchise difficult in itself, that is you adding difficulty to something that originally isn't, which is different. My yt channel is full of sl1 no damage lvl 1 weapon of all the souls trilogy, should I talk about its difficulty using this as a standard of how the games are meant to be played or can I assume that the games proposal is radically different from this and these are just rules i've added on my own accord and shouldn't be representative of the game's original difficulty?