I find Marriott responds far more quickly than SPG's BRG ever did (SPG seldom came back within 24 hours whereas Marriott almost always does). However, in addition to the issues discussed above I'm finding it basically impossible to predict when a claim will be successful or not with Marriott. It's not as bad as, say, IHG who basically just try to come up with any possible reason to deny the claim, but it still seems very random as to how the rules are applied each time.
Case in point - just did an LNF for the W Doha. Found a much cheaper rate than direct on Amoma but given that Amoma doesn't properly specify the room type I was worried Marriott would be pedantic and deny the claim even though I was booking the cheapest room type, as this happened to me last time I tried to make a LNF claim. I therefore also submitted a second claim for a slightly more expensive rate including breakfast that I found on Booking.com against a second booking including breakfast, which I was fairly sure would get approved instead as a back-up. What happened was, of course, the cheaper Amoma claim was approved almost straight away (so why was my last Amoma claim denied for that issue?), whereas the Booking.com claim was denied (because there was a cheaper rate available on the property's website which was only available to holders of Qatari ID). Yeah, sure.