That depends on how it is handled. Bards get to choose from every spell in the game. Does that make them broken?
Access to any spell in the game at once is different from adding two to its list. And its is a very, very powerful feature, make no mistake. Being able to have any spell in the game, like the 3e version some are asking for, is more powerful than the Bard's version. It'll have to be "watered down" and limited, simply for the sake of not overshadowing the other classes; that's seems to be a sticking point for some people.
One thing about the 5e classes -even the subclasses- is that each is unique. Even the necromancer-styled cleric, wizard, and Oathbreaker all feel and work differently from each other, giving vastly different playstyles. The Bard gets access to other class spells only at milestones, and only in limited quantities. The Tome Warlock gets access to other rituals and limited cantrips. The two don't overlap.
For an artificer, that means that any cross-class casting will need to be markedly different enough from our previous examples, but still balanced, so there's neither overlap, nor overshadowing. Both are critical.
Yet we have Mearls posting on ENworld that artificer could be its own class:
Make no mistake, I never said it wasn't possible.
1) I'm opposed to the idea that its entirely on the D&D teams' shoulders.
2) That the subclass features were created so that we could avoid class bloat. Making the artificer its own class requires greater weight than any of the core classes, which are "grandfathered" in.
I'm simply saying that going "Well, the Ranger or Paladin got in!!!!" is a false comparison, a distraction, that doesn't matter. The artificer, as its own class, needs to stand on its own two legs with its own design plan. You want a more robust class? Then we should be talking about ideas here. Simply saying "I want more, go make it for me!" is disingenuous.
I
don't find the argument of "create magic items" to be a strong one for a new class. Technically, we have rules for every class to create magic items. They're optional, yes, but that's something that will likely be turned on in Eberron by default.
As well, if we're discussing putting Rune Magic under the Artificer class, remember that the Artificer won't be Eberron only at that point. Rune magic is a classical dwarven style of cleric that's been around for a long time in a lot of settings. So the Artificer will need to be something that fits into the general game, even in low magic item settings. That's going to have to be a consideration as well. Which is why I suspect it won't make it as an artificer, because fans will want it as its own class that shines in the Eberron setting, but doesn't fit anywhere else.
I should add - I say all this as someone who has never played an artificer or seen one in play. I'm just noticing that the argument that they should be a wizard sub-class seems to come predominantly from those who aren't artificer fans. While the predominant view of fans seems to be that, for the sorts of reasons @
ThirdWizard gave, they don't fit under the wizard's umbrella.
For the record, I don't think they fit perfectly under the wizard either. I don't think that it fits under any class at the moment. Part of the reason I'm actually advocating a series of subclasses; that, and I admit to being excited over running a game with the entire party with access to artifice in some way. I could get the people interested in a grenadier fighter, magic gunner rogues, alchemist bards, golemancer wizards, and a rune cleric with an artifice theme going, but no way I could get them to all play a single class, even if they'd all be different subclasses.
I admit to being the minority here. No one seems to be interested in discussing that idea. This thread seems to be highly polarized with few discussing other options.
I get the feeling that too many just aren't interested in anything BUT the 3e class. I hear talk about it getting ways around the built in limitations of casters, avoiding Concentration and the like. That's not something I think will work.