I can't comment on long-distance reliability etc. - I simply do not have experience some great travelers here have. I never been to Mongolia, but similar places and not on bike.
But I can comment on "working on bikes" (I am "hobby mechanic" and my friends with their bikes abuse that )
I worked on 3 transalps (600, 600, 650) and 1 Vstrom 650 post-2007 model, can't recall exact year. I also did a bit of "bolt on" work on TA700 and test rode it.
I wouldn't say that Transalp is easier to work on than Vstrom. It's quite densely "stuffed" around engine, there is some questionable components locations, it's not light on amount of wiring, it has plenty of plastics (just like Vstrom)...
Vstrom are tad easier, because of fuel injection takes less place and cylinders are 90 degrees and overall it is a bit more...spacious and simpler built than TA.
TA engine thou is silky smooth and reliable, but Vstrom 90 degrees twin definitely packs more punch.
As for TA700 - I know some people will not like me for saying that but I hate it. For me it is symbol of Honda turning into different kind of bike maker. It's INCREDIBLY heavy for what it is, it does all average and it is nightmare to work on. I prefer old 600 for that matter.

However, for dirt run through Mongolia I would never take any of those but rather simple DR650 or Tenere 660 or (in my case) my modified DR800. Then Transalp 600 (because it's cheap) and then Vstrom 650 because it is brilliant all-rounder, thou heavy (partially can be fixed by ditching stock exhaust system).

I don´t hate the 700, in fact I actually do like it.

But it is heavy as you said...... and I must admit, I found it kinda surprising, that when it came out, it was already behind the DL650 in some key areas mentioned before. And nevermind the DL – now the buyers also have BMW 650/800GS and Triumph Tiger to choose from. At least in my country, the XL700V and 650GS (800cc) ABS-versions cost exactly the same.

Still, I´m even more baffled at Honda´s decision to not update the Africa Twin and just plain quit such a great bike!

well... if you ask me great art of monster trailies got lost after "speculate on dakar wave" ended after AT, S10, DR800, Elephant etc. I'd love to see my DR800 contemporary version (alike Tenere 660 2008 model) - with injection, better suspension and brakes etc.
So I build one for myself..except injection. I see no bike on market to change it to...unless Morini Granpasso/Granferro will be revived in big-scale manufacturer with good dealer network etc.