Broken Base: On the Radiant Mythology side, people are split on who the best healers are. The arguments usually are whether or not Estelle, Mint, or Tear is the best healer, nevermind the fact that the above three are only a few of the healers in game.
In general bringing up ideal parties is treading on Flame Bait.
Ensemble Darkhorse: Kanonno Earhart, The heroine of the second Radiant Mythology game. She made it into the 5th Tales Of popularity poll, beating out fan-favorites like Asch and Spada, which is pretty uncommon and quite a feat for an escort title character. Since then she has been a recurring character in Drama CDs, Tales Of Link and even played a heroine role in the second part of the TalesOfAsteria game trilogy. Not bad for a main character from a spin-off game.
What makes her notable is that neither her predecessor nor successor managed to make it to her level of popularity among the Tales Of fandom.
Game Breaker: The third game introduces such a large cast that many just make the game a cake walk.
Van. He's a Glass Cannon sure but he's got amazingly high magic attack and all of his magic artes are high end spells like Judgement or Frigid Coffin.
Emil also too. He's got great speed and yet really damn powerful artes.
Radiant Drive in RM3. It's basically Vesperia's Overlimit that locked you away from using Mystic Artes. With it, you can replicate BLAH BLAH BLAHTIDAL WAVE here.
Fog in Narikiri Dungeon 3. Every fight can be won by just spamming Elemental Master.
Germans Love David Hasselhoff: Tales of Link is far from well received in native Japan (Due to it's sheer difficulty and gatcha rate and various bugs), In the localized server however they are more well received.
No Export for You: All of the games in typical Tales of fashion, save the first Radiant Mythology game and Tales of Link, have not been localized, although all three Kanonnos, to name a few elements, have appeared in the global version of Link nonetheless.
Tier-Induced Scrappy: Since each game has such a large amount of characters, some are not very useful.
Chat is one because she has four artes, none of which are really strong. The only useful thing about her is her ability to spam attacks in Radiant Drive.
Asch too is one mainly because he's a Master of None. He's a Magic Knight sure but he has no highend spells, no healing, and he's a bit clunky. Luke has most of his artes but is stronger, Jade is a better spellcaster, and Van is a Magic Knight with much better artes and stats.
Judith, which is sad because it's only because her artes were implemented in weirdly and she can't combo very well.
Annie, mainly because her fighting style is by making glyphs that either buff or heal...which is a issue when you have a 3D style that makes it IMPOSSIBLE to use it.
Between the Descender's classes themselves, Thief and Archer. Especially Archer post-RM2 due to Nerf.