Objective Secured Death Company Dreads? Ace.

Quick go through of the flyer aces. In short, absolute gash for most unless it's free as a result of the special mission. I can't see myself ever picking up the points cost for these upgrades.

For the price of a Rhino, you get a d3 random roll for a bonus to the flyer, unless it's a mission with the Fighter Ace special rule, which grants a single Flyer/FMC a free roll. Note, RAW, it doesn't affect Super-Heavy Flyers or Gargantuan Flyers either.

Astra Militarum/Militarum Tempestus
Benefits; Come on from any edge for reserves, +1 to Front Armour, 12" Fearless bubble
Affects; Valkyrie, Vendetta, Thunderbolt Fighter, Lightning, Aquila Lander, Vulture Gunship, Valkyrie Sky Talon, Arvus Lighter, Avenger Strike Fighter
Comments; more armour never hurts. With the Valkyries and Vendettas getting AV13, they are very resilient flying tanks - Icarus Lascannons need a 4+ to glance, and good versus Autocannons. The coming on from reserves is different from all others. It's not ongoing reserves only, it is flat out reserves - and it's a choice. Vendettas are excellent with this. The weakest option is a 12" Fearless bubble - and with average Ld7, that's never a bad thing. It benefits the dedicated tank hunters in particular.

Blood Angels
Benefits; +1 BS, 180 degree pivot before move, 1/game 12" preferred enemy bubble
Affects; Caestus Assault Ram, Land Speeder Tempest, Storm Raven
Comments; +1 BS with Blast weapons never really helps. 180 degree pivot can be useful to position for a perfect table grab on a zoom however.

Chaos Daemons
Benefits; Rerolls failed to wound/armour pens when Vector Striking, +1 Wound, +1 Invuln
Affects; Bloodthirster, Lord of Change, Flying Princes. Be'lakor, Fateweaver, Blight Drones of Nurgle,
Comments; Vector Strikes suck. Bloodthirsters suck in 7th. +1 Wound is always useful. Blight Drones don't Vector Strike, have no wounds, and would only get a 6+ Invuln. Special Characters would do well. Fateweaver has a decent chance of getting the +1 Invuln, and with Cursed Earth+Grimoire can get a 2++ Rerollable.

Chaos Space Marines
Benefits; +1 BS, Reroll failed to wound/armour pens when Vector Striking, 12" Relentless Bubble
Affects; Heldrake, Chaos Storm Eagle Assault Gunship, Hellblade, Dreadclaw, Hell Talon, Blight Drones of Nurgle
Comments; Helldrakes often run with Flamer, making BS useless, other options are Vehicles not creatures so no Vector Strike, and the 1/3 chance of a Relentless bubble, despite being fairly powerful isn't exactly to be relied upon when for the cost of a Rhino you can give a unit a Rhino and move them nearer.

Dark Angels
Benefits; +1 BS, Come on from any edge for ongoing reserves, 12" Deep Striking no scatter bubble
Affects; Nephilim Jetfigher, Dark Talon, Storm Eagle Assault Gunship, Caestus Assault Ram, Land Speeder Tempest
Comments; +1 BS with lots of Blast Weapons or TL doesn't make that much difference, a change of angle for ongoing reserves is good if used, but not worth the cost, while a no scatter bubble, despite being huge is available for cheaper.

Dark Eldar/Eldar
Benefits; +1 BS, 180 degree pivot before move, 12" Hatred bubble
Affects; Hemlock, Crimson Hunter, Voidraven, Razorwing, Phoenix Bomber, Lynx (special case), Nightwing Interceptor, Raven Strike Fighter
Comments; +1 BS is helpful unless TL'd, pivot just makes them that much more agile, and the Hatred bubble is okay if you're in an assault. Assault sucks, but this makes you that little bit more deadly when you get there. However, if you're in an assault with these fragile armies, the addition of hatred should matter so little that you shouldn't be banking up the cost of 3 exarch warriors on the cost of this 1/3 upgrade.

Grey Knights
Benefits; +1 BS, +1 Cover Save (Max 3+), Psychic Pilot Mastery 1 (Daemonic (Sanctic) only)
Affects; Storm Raven
Comments; Lots of TL'd/Blast weapons anyway makes that less useful. +1 Cover save makes it even more survivable, and actually something to risk hovering if transporting troops for a change. I don't play GK's or Sanctic for me to judge how useful that is, but additional warp charge is never bad if you're relying on that I guess.

Necrons
Benefits; +1 BS, IWND, +1 to Reanimation Protocols up to a max of 4+
Affects; Nightscythe, Doomscythe, Nightshroud
Extra BS is always good, and IWND is never bad on decent HP vehicle, while Reanimation Protocol bubbles is never bad. Question is, at the cost of 1/3rd of the price of another Nightscythe it might be preferable to just take another for double the HP and damage anyway.

Orks
Benefits; +1 BS, +1 Front Armour, 12" FNP 6+ bubble
Affects; Fighta-bommer, Attak Fighter, Codex jets I can't remember names of.
Comments; TL'd and higher BS puts them on Marine level accuracy. Enjoy. Armour I can't remember, but is still paper thin - benefits by making S4 shots ignored though. FNP bubble is ace, however. With several huge mobs of boys, passing just 6 6+ saves in a game makes up for the points it costs. However, taking 6 more boyz might be more preferable. YMMV.

Space Marines
Benefits; +1 BS, 180 degree pivot before move, 1/game 12" Preferred Enemy bubble
Affects; Storm Eagle Assault Gunship, Caestus Assault Ram, Land Speeder Tempest, Storm Talon, Storm Raven
Comments; +1 BS with TL BS4 weapons isn't a massive increase in damage, 180 degree pivot move is ace as ever, and 1/game PE bubble is okay, but expensive. Might be interesting in an IF list, but no guarantee.

Space Wolves
Benefits; +1 BS, Monster Hunter, IWND
Affects; Storm Eagle Assault Gunship, Caestus Assault Ram, Land Speeder Tempest, Two new flying coffin things
Comments; Tl'd BS4 and Blast Weapons not so useful. Reroll to wound versus MC's is ace, as is IWND though.

Tau Empire
Benefits; +1 BS, +1 Cover Save (max 3+), d3 markerlights to one enemy unit
Affects; Barracuda Air Superiority Fighter, Remora Drone Fighters, Sunshark Bomber, Razorshark Fighter
Comments; These are ace options for all. Additional Markerlights makes up for stealing the FA slots of pathfinders.

Tyranids
Benefits; Autopass Instinctive Behaviour bubble, +1 Wound, Can enter ongoing reserves if within 12" of table edge at start of enemy shooting phase.
Affects; Harpies, Crones, Flyrants
Comments; Is the need for Instinctive Behaviour cover all that important to you? Why not take more Synapse? Additional wounds are always good, and just removing a unit at risk from enemy shooting if near a table edge is brilliant. Not sure it's worth it, though. Might have some dodgy interactions with the Skytyrant formation from Shield of Baal.

Any other faction
Benefits; +1 BS, +1 Cover Save (max 3+), 12" Fearless bubble
Affects; anything that's not explicitly above, which includes the FW lists and the HH armies. Codex supplements use the core army, but FW lists are their own unique thing despite referencing the dex. If using this rule, recommend double checking that you can use the rules with obvious links (i.e Tyrant Legion uses SM Dex and flyer upgrades, Maynarkh Crons use the Necron, etc. HH use SM or the relevant chapter options. If not, the options are too broad. In general, most of these suck.