Why would I personally use a Toyota over an Iveco / Unimog? This is an interesting question, which requires a slightly lengthy answer...so please bear with me.
First off, in Pakistan, Toyota is King. Especially the Land Cruiser. If you refit a Fiat (Iveco) or Mercedes (Unimog), you will still never be able to match the availability of parts, reliability, and economy of Toyota. Here, I think that Land Rover, Range Rover, Nissan, Mitsubishi etc...all come very very far behind, in my opinion. Unimog and Iveco on the other hand are RARE. So if you're headed out off the beaten track...be prepared for being left high and dry if something catastrophically fails.
Secondly, driveability. I hope Iveco and Unimog fans excuse me for saying this, but the amount of wear and tear that a Toyota SUV will take is unrivaled and it manages to do so, effortlessly. Whether you're driving on a highway, or through a mountain pass, or snow, or ice, or sand...Toyotas are unbeatable. The comfort levels in terms of cabin, seats, heating / air-conditioning, ride quality, and engine / road noise in a Toyota are pretty much unsurpassed.
Third, flexibility. I think that the number of combinations for Toyota engines and gearboxes and transfer cases alone, are overwhelming. You have a dozen options for Diesel alone. Maybe two dozen for petrol engines. You have ten different axle ratios, to cater for economy, power, uphill climbing, desert bashing, and highway cruising.
The Iveco and Unimog don't even begin to approach any of those aspects, unfortunately. So (personally) I would NEVER create an expedition rig which is supposed to transport family and loved ones, across diverse and difficult terrain...based on either of them. They're simply not reliable enough, not speedy enough, not economical enough, and not driveable enough, in my personal opinion.
Fourth (and most importantly), I love Toyotas.