I have stayed about 7 times after ambassador ( extra 4 stays after reach ambassador tier but not yet assigned any ambassador)
From all those stays after ambassador, 2 not upgraded due to the room booked are the standard suite category, 1 due to some visiting dignitaries fully book the hotel, 1 suite upgrade, and the remaining 3 is upgrade to higher room category.
(the 4 stays prior to ambassador being assigned : 1 no upgrade as the hotel was an all villa resort, 2 upgrades to higher room category and 1 no upgrade as the hotel being stingy
)
And about Accor luxury hotels, I agree completely that their offering still below what Marriott offers especially in certain areas.
Take Japan for example, most we can get there is Ibises and Mercures ..... while Marriott have some St Regises RCs and Luxury Collections around. And some of them are really amazing property like Suiran and The Prince Gallery (both from The Luxury Collections hotel)
In Singapore, Accor have slight advantage here as the Ritz Carlton Sg is not participating Bonvoy property. so Marriott only have St Regis Sg as their luxury hotel while Accor have Raffles and Fairmont.
Another advantage that make me choose Marriott so far are their aspirational properties around the world.
Al Maha Dubai, The Ritz Carlton Ras Al Khaimah Al Hamra and Al Wadi, and Suiran Kyoto cant find Accor equivalent for these properties in their respective area