First seen in BW Block (though in spirit, the first ACE SPECs were the Pokémon ☆ from EX Block), ACE SPECs are powerful cards, but you can only play one ACE SPEC in your entire deck. ACE SPEC diversity in BW Block was not great (most decks just played either Computer Search or Dowsing Machine), but this has been corrected in SV Block.
All decks should play ACE SPECs. (This is a key difference between them and Pokémon ☆.) Even one of the worst ACE SPECs, Master Ball, can improve any ACE-less deck by replacing a Nest/Ultra Ball, so ACE-less decks are never optimal.
No matter what card type your ACE SPEC is, there's always something that can search it out:
Dragapult ex:
Pure/With Dusknoir: Neo Upper Energy
With Charizard ex: Unfair Stamp
Gholdengo ex: Energy Search Pro
Raging Bolt ex: Prime Catcher
Gardevoir ex: Secret Box
Charizard ex:
With Pidgeot ex: Maximum Belt, Secret Box
With Dusknoir: Precious Trolley
N's Zoroark ex: Secret Box, Unfair Stamp
Tera Box: Sparkling Crystal
Marnie's Grimmsnarl ex: Secret Box
Flareon ex: Sparkling Crystal
Joltik Box: Prime Catcher
Armarouge Box: Legacy Energy
Conkeldurr: Enriching Energy
Ethan's Typhlosion: Secret Box
Crustle: Poké Vital A
Ceruledge ex: Brilliant Blender
Miraidon ex: Prime Catcher
ACE Nullifier prevents your opponent from playing their ACE SPEC if Genesect has a Pokémon Tool attached. Keep the second part in mind; you need to be playing Pokémon Tools (that aren't TMs, since those discard themselves at the end of the turn) for Genesect to do its thing.
Tool Scrapper beats ACE Nullifier by removing the thing it's dependent on.
Note that Jamming Tower does not stop ACE Nullifier from working. ACE Nullifier only cares that Genesect has a Pokémon Tool attached, not what the effect on that Tool is. Jamming Tower can actually end up helping Genesect, as it stops TMs from falling off at the end of the turn, providing more potential Tools to turn on ACE Nullifier.