DL does apply the airport benefits (checked bag, Sky Priority) to all passengers on the reservation (likely a Deltamatic limitation).

The Comfort+ upgrades are global on DL (i.e. also on long-haul). Platinum does now clear on ticketing (note that DL holds two Comfort+ seats for gate assignment: you can clear the upgrade and be standing by for a seat), Gold can clear at T-72, but at a priority behind all of Delta's Gold and higher flyers (and at T-24, FB Gold will be behind DL Silver and even non-status who hold the Reserve Amex). FB Gold and Platinum will also appear on the upgrade list for F (including international flights where DL has F) and domestic Premium Select/Delta One. These upgrades only become possible at T-24 and prioritized behind all DL elites and Reserve Amex holders (viz. very unlikely to clear).

IMO FB only makes sense for domestic US flights if you WFBF (note that taking DL's buy-up offers generally won't affect your FB earnings: you need to go book First from the start or go through the "upfare/change flights" process for the F to earn in FB... this might not be possible on trips booked through work).

KE's SkyTeam benefits are generally paltry.