Artificers with the Magic Item Savant Feature only require Ability Checks to cast spell scrolls if it's a 5th level spell on the Artificer Spell List, and the Artificer is below level 17
Under normal conditions, a spellcaster is unable to cast (or even read!) a spell scroll bearing a spell that isn't on their spell list (Spell Scroll, Dungeon Master's Guide, pg. 200). However, as has been established, the Magic Item Savant feature allows them to ignore any class or level requirements to use a magic item, which includes spell scrolls.
Then, the requirement to make an Ability Check is worded like this:
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully.
Since Artificers gain the Magic Item Savant feature at level 14, the only spells that will ever fulfill this condition are 5th level Artificer spells. Any other spell is
- Of a level they can already cast (so the condition doesn't apply), or
- From a Spell List for a different spellcasting class (so the condition doesn't apply)
Therefore, the only time an Artificer with the Magic Item Savant feature ever needs to make an Ability Check to cast a spell scroll is if they are between levels 14-16 and attempting to cast a Spell Scroll for a 5th level Artificer spell.
"But that seems... really specific. Especially since it also means they could cast 5th level spells from any other class' spell list without the check!"
Yes. At my table, I'd probably houserule it to simplify and just say that any Artificer with the Magic Item Savant feature no longer requires Ability Checks for any Spell Scrolls. The slight boost in power is worth, in my opinion, the time otherwise spent by players and DMs double-checking to determine if a 5th level spell is on the Artificer Spell List or not.