To further add to this, even on Digicel's highest end plan (I think it's the MaxSmart plan) which allows 10 GB per month, they engage in heavy handed leveraging of the FUPA on subscribers of that plan. 3G is also affected by the number of subcribers on it and will "breathe" i.e. decrease its coverage geography to provide better service to those well within the cell or the site will ask the User Equipment (your phone) to do a reselect to 2G. Digicel is particularly affected by this and therefore they see it necessary to curtail any "super-users" on their network despite them going to DC-HSDPA (42 Mbit/s max) in Kingston and MoBay from what I've seen.
In terms of backhaul capacity, I can only speak to FLOW's network capacity. Where they haven't fiber backahul, they use IP/MPLS or Alcatel-Lucent MPR 9500 microwave radios that provide up 1 Gbit/s on a single carrier (which they might be using currently, they can scale/aggregate the carriers to increase that capacity as required) or even Ericsson radios that perform just as well. You'd find the microwave radios used in rural areas or dense urban rooftop sites. Hub sites in MoBay & KGN are all fiber fed. I've seen FLOW's Huawei equipment (North Coast) that aren't even fiber do DL speeds up to 18 Mbit/s when it has no network load, given the network's theoretical limit is 21 Mbit/s.... it's insanity.
FLOW's speak of lack of capacity originates from capacity issues with 3G technology (which was never meant originally to be a data bearer, only improved voice was conceived). They whitelist their company developed apps as well as Wikipedia but unlimited everything would reek havoc on the network resources. One 3G cell site can only serve around 360 in URA-PCH (i.e. the most low-powered state your phone's HSPA radio can go into - no data transfer taking place), having unlimited users would cause huge slowdowns and worse, a network faliure.
Once we've gone LTE, a network purpose built for data (no cell breathing, 3x user capacity etc.), we will have unlimited plans... Let's just be ready to pay that hefty price lol.