There are a few units throughout the Warscrolls which have synergies not solely within their own factions. Though not as many as first thought, and nothing I can see as being game breaking.
The main areas of synergy are Brettonians and The Empire with the Free People keyword, Vampire Counts and Tomb Kings with the Death keyword, Chaos warriors and Daemons with Bloodbound through the Khorne Keyword and finally Chaos warriors and Daemons with Skaven with the Nurgle keyword.
There are a few other odd occasions where units can affect other factions with generic keywords like War Machine and Monster, but not very many.
One thing I haven't included in this post is units with abilities that affect unit with no keyword requirement. I plan to go through and update this post with a separate list.
Below is a list of units with external synergies:
• Karl Franz and Kurt Helborg, King Louen, Fay enchantress and Damsel of the lady buff free people.
• Empire Greatswords and Knights of the Realm are buffed by a nearby free people hero.
• War altar and Celestial hurricanum buff free people.
• Luminark of Hysh buffs order wizards and free people.
• Dwarf engineer and Grim Burlocksson can repair any war machine.
• Dwarf Thane BSB has a negative synergy with all friendly wizards.
• Prince Imrik's command ability affects all dragons.
• Dark Elf Beastmaster buffs any monster.
• Seraphon, Wood elves, Stormcast Eternals and Fyreslayers have no external synergies.
• Bloodbound Bloodstoker and Bloodsecrator buff any unit with Khorne keyword.
• The glottkin, Orghots Daemonspew, Putrid Blightkings, The Great Unclean One's plague wind and Epidemius' Nurgles Tallyman affects all Nurgle.
• Vermin Lord corruptor, Lord skrolk, Plague priest, Plague furnace, Plague monks, Plague censer bearers and the Plagueclaw catapult all have the Nurgle keyword and can be affected by the above Nurgle units.
• The changeling and the Blue Scribes gains spells from any wizard.
• Great bray shaman summon any monster from any faction or grand alliance! This includes heroes with the monster keyword.
• Tamurkhan's command ability affects all monsters.
• Chaos war mammoth buffs marauders and marauder horsemen.
• Throgg buffs all chaos monsters.
• Chaos Familiars buff all chaos wizards.
• Legion of Azgorh have no external synergies.
• Ogre Kingdom and Orcs & Goblins have no external synergies. O&G have especially segregated internal synergy between the individual factions.
• All death wizards can summon all death units.
• Settra buffs Death units and Deathrattle.
• Necrotect and the Royal Warsphinx buffs deathrattle.
• Tomb herald buffs Deathrattle and is a wound shield for any death hero.
• Liche priest spell affects skeletons or reainimants.
• Khalida, Krell and Tomb Kings buff skeletons.
• Skeleton warriors buffed by death hero.
• Nagash buffs death and gains all spells from any death wizards.
• Manfred Mortarch, Vlad, Vampire Lord, Mortis Engine, Wight King, Zombie Dragon and Heinrich buff death.
• Necromancer is buffed by nearby death units and their spell affect skeletons.
• Corpse cart and Arkhan buff death wizards.
• Skeleton warriors buffed by death heroes.
• Vargulf and the Black Coach buffed by death wizards.