I don't know if anyone's noticed or mentioned this anywhere yet, and I hope errata doesn't eventually come out that negates the stacking of these, but... I was playing around in the creator and realized that a 20th-level artificer can stack 6 of the class attunement attack/DC items for a spell attack modifier and save DC that'll make your DM do a spit take.

I don't think Errata is necessary to prevent stacking of these things. Note each one applies to specific class spells:

All-Purpose Tool specifically applies the bonus to Artificer spells.
Arcane Grimoire specifically applies the bonus to Wizard spells.
Bloodwell Vial likewise to Sorceror spells.
Moon Sickle to Druid and Ranger spells.
Rhythm-Maker's Drum to Bard spells.
Rod of the Pact Keeper to Warlock spells.

Magic Item Savant states:
You ignore all class, race, spell, and level requirements on attuning to or using a magic item.

Wait, hold on. It says using in addition to attuning... hmm... Suddenly I'm far less convinced of my original position.

Edit:

No wait. I think I figured the thing that doesn't let all of them stack. Most of them require you to be holding the item, as in, holding it with your hands. Which brings you down to just two of such items.

In that specific list the only one that lets you wear the item is the Bloodwell vial. So that's a +9 bonus to Spell Attack and Save DC instead of a +18.

Amulet of the Devout is another one you could wear to gain a bonus to spell attack and DC so you could manage +12. Leaving two attunement slots left for furthering this madness one of which could got to Robe of the Archmagi granting a +2 more to DC. Leaving you with an absurd +12 to spell attack and +14 to your DC.

I think the bigger limitation here is that you're using 6 very rare items - speciffic ones at that. I don't have Tashas yet (damn delay for europe...) but if what Unclevertitle has it right and the items give bonusses to your bard/druid/etc. spells then you won't get a stacking effect. As I read Magical Item Savant, you can attune and use the magic Item but it doesn't mean that a item that gives you i.e. +3 for your warlock spells gives you +3 to any spell now.

Yeah, I kind of agree with Sutlo.  You could definitely attune them, and gain their benefits to all of your bard, ranger, wizard, etc. spells, if you have those.  The effects would not stack to your artificer spells.

This is clearly a case of the DDB digital sheet not being as specific as it should be.  While Unclever is correct in that there's a potential argument, RAW, for this ludicrousness, I can't imagine many DMs who've been running a game for more than three sessions will allow an artificer to use items meant to bolster another class's spells to turbospike their own modifiers.  And if the artificer in the party does start hogging all these items for herself, the rest of the party watching the artificer with a half-dozen purple-grade items megabloating their spell modifiers is not really in the wrong when they mug that character in her sleep to get their share of the party loot back.

Quote from Yurei1453 >>

I can't imagine many DMs who've been running a game for more than three sessions will allow an artificer to use items meant to bolster another class's spells to turbospike their own modifiers.

I think DMs should allow that. The artificer has an entire feature dedicated to using items they normally wouldn't be able to. Let them use it. DMs shouldn't be in the business of nerfing away entire features, and it's not as if artificers are in need of nerfing.

Quote from Unclevertitle>>

Wait, hold on. It says using in addition to attuning... hmm... Suddenly I'm far less convinced of my original position.

If you ask me you had it right the first time. Ignoring restrictions on the item's use doesn't mean the artificer can do the things the item is designed to benefit. Just like a cleric/wizard won't gain the grimoire's benefits when casting cleric spells, an artificer attuned to the grimoire won't gain the benefits while casting artificer spells. Likewise an item that improves a cleric's Turn Undead won't do anything for an artificer that lacks that class feature.

Most of these items say that they add to the attack roll and DC of your [Class] spells.  Druid and Ranger for the Moon Sickle, Bard for the Rhythm-Maker's Drum, and so forth.

You can attune to the item as a high-level artificer, but that attunement does not change what the item does.  It provides a bonus to your [X] spells.  That's not a restriction on what can use the item, it's an explanation of what the item does.  If you happen to have spells from the class without being the class (a'la Magic Initiate), then you gain the bonus to those spells.  All of your spells are still artificer spells, which do not benefit from boosts to druid spells, ranger spells, warlock spells, or anybody else's spells.

It's the same situation we had before with the Rod of the Pact Keeper.  Yeah, a high-level artificer can attune to a Pact rod, but that doesn't mean it has Pact Magic spell slots to recover or warlock spells it can cast to receive a boost.  Not unless it's also a warlock, at which point the question is moot.

At any rate, it opens up an interesting avenue to multiclass into a non-INT spellcaster without necessarily having the stats to back it up. Dipping into bard for some spells seems a little more viable if you can use that +3 to bolster your success rates.

I think the real limiter to an artificer bolstering their spell attack and spell DC with this method is that you have to be using the item in question as an arcane focus in order to gain the benefit of the bonuses, and trying to say you should receive all the bonuses from all the arcane focuses you have on your person for one spell is like trying to argue that all your weapon attacks should get a +12 to attack and damage because you have a +3 shortsword in each hand, a +3 long bow strapped to your back, and a +3 dagger in your boot.

For the “All-Purpose Tool“, it states that it only provides the +1 to saves and attacks for artificer spells - not all spells.  The moon sickle should only work with druids.

This isn't something for errata to fix - this is a bug in the character sheet.

As many of you has already noted, it doesn't work straight that way. Even with the keyword using, some of them applies to specific brand of spells, and some has to be held.

You can, however, attune to those four bad boys to crank up to a +22 to hit and 27 spell save DC.

Well here is another question.

Can you just attune to a +1 +2 and +3 version of the All Purpose Toolss, and gain a +6, and 3 on demand cantrips (primal savegry in my case--that really should've been on the list ITS AN ACID LIQUID BLADE!!!). 

Is there stacking rules against that in 5e?  I don't really know.  but would be an interesting way to get a few early on.  I was wanting to get two +1s simply because I wanted primal savegry long term.

I still wish we had the UA Armour of Tools. Would make holding nad using that as a focus a lot easier for my Alchemist instead of Item Juggling.

Yeah, the concept for armor of tools was nice. I didn't really care for how it used up my armor slot (for lack of a better phrase) for infusions, but always having a tool on hand just fits the artificer.

Honestly I feel like that was pretty heavily created to 'fix' the issue with the alchemist and handedness compared to the other subclasses..

but it was fantastic for truly making you the master of tools. Which I was fond of.

True though I feel like it would make a good non armour piece of infusion

Regarding the four item build shown above: In theory yes, but you can't be holding anything else, and the DM who lets this happen needs to be smacked.  I'm having to convince my DM to let me make a +1 All-Purpose tool, as he thinks since it enhances my spell save it's too powerful.