I'll say the World of Mana games tended towards being just eh, but what's really annoying is they all just needed a bit more work. Except Children of Mana. That one actually felt complete but it was so basic, and the plot was nothing special. Heroes of Mana actually had a really good plot, but it was blatantly a tech demo for Final Fantasy XII: Revenant Wings. And Dawn of Mana...that really needed gameplay polish, and maybe changing the level up system so that you still collect the tokens, but you don't start from square one every single area.
When you put the whole series together, though, the Mana Goddess is incredibly weak and incompetent. Or Mana Goddesses I guess. They keep dying and the one time she actually does something active, it results in the Scion of Mana and a Mana Storm threatening the entire world. Hell, she only directly gives the Mana Sword to someone maybe once. Otherwise, the characters keep finding it stuck in a random stump somewhere.
Children had that stupid physics thing that Dawn had that made
doing anything in that game a royal pain in the ass at points (particularly in rooms filled with spikes, ice, and other obnoxious floor tiles). The idea was to get your allies to knock enemies towards you so that you can then knock them towards your other allies and so on, but of course this ignores the obvious flaw of having four people dumb enough to actually sink money on this game in close proximity to each other (especially since the game had absolutely nothing else going for it). The plot being a direct sequel to Dawn doesn't help matters since it's only a sequel in a chronological sense (the only returning character that isn't a series mainstay was the evil Sword of Mana from Dawn, because there's a totally evil Sword of Mana now). Out of all the action dungeon crawlers on the DS, Children is probably the second worst of them all.
Heroes was magnificently dumb since its an RTS made with Squeenix's difficulty design philosophy of 'give everything bigger numbers' (granted, due to the nature of the game and it's wonderful pathfinding, the only other option would have been to go for a Starcraft style Zerg Rush). And while the game tries to pretend that its an RPG, its not and your characters and units will get left behind in the power curve, either slowly or rapidly depending on how aware you are about the treasures hidden on every map and how much effort you expend in getting them and the ranked treasure. It's also a prequel to SD3 which wouldn't be so bad, if SD3 had ever left Japan properly (at least it got a fanslation). And it also used the now defunct DS WiFi to access bonus missions to grind for gear in (in other words, you can no longer grind in a game that basically requires grinding).
Friends never came out here, and I've already talked about Dawn (but I will add that you can get around the whole 'start every chapter at level 1' thing with the game's medals, but anything beyond the most absolute basic stuff is a pain in the ass to get).