Tourers benefit from triples, and if you're going to run a triple, 3x10 doesn't offer much on top of 3x9.
There are two technical reasons to prefer triples. The first is gear range:
- A typical 1x11 setup gives 11-42T at the back, for a difference of 3.8:1
- A 2x10 or 2x11 drivetrain might come with 46/30x11-30, for a difference of 4.2:1. That's GRX, meant for gravel bikes - road setups are geared a little higher but have a similar range, though I now have exactly that range in 105 on the road wheels for my gravel/endurance bike, Some 2x10 mountain setups go wider, as do 2x11 gravel (I have an 11-34 cassette on my gravel wheel).
- The stock 3x9 Sora on my tourer was 50/39/30x11-32, for a range of 4.8:1 (and you could put 11-34 on the back easily, stretching this still further)
You may well be slogging up hill for long periods, laden, so you need low gears. The top end on my tourer sees less use but there are certainly sustained descents where it's useful.
The second, that has been a great help to me, is fault-tolerance. There are 2 factors here:
- A 9-speed derailleur works tolerably well even if not set up perfectly and it's easy to index. Even with a slightly bent hanger or axle you can get a good range of gears without ghost shifting (when my axle was bent, I could get both ends OK, but had some ghost shifting in the middle of the cassette. If you trash your derailleur and can only get something even more basic, you'll still be able to ride even if it's not perfect.
- Parts break unexpectedly. Normally this is cables. My RD cable usually lasts around 5000km, but has been under 4000km. I get about 50km of warning, when I think "that's funny, I adjusted it not long ago". Bike shops can be much further than that apart, and carrying a spare inner doesn't always solve it as the outer can be damaged. But with a triple, you can lock off the RD and get quite a long way on a wide-ratio 3-speed. I've done 200km that way. Since then I've done something similar after messing up changing the cable and jamming it in the shifter - 500km of a recent tour was done on 3 speeds (though I selected a sprocket for the riding conditions each day).
Closely related is durability of all drive-train parts, but that's been addressed in other answers already.
In addition to the technical reasons to like triples, look at the availability around brakes.
High end groupsets normally come with hydraulic braking. Decent hydraulics can be very nice, but cheap ones are horrid (I've got some on my MTB and I'm seriously considering getting rid of them - performance is poor and they're a pain to service; I may even have a leak). But even good hydraulics have failure modes that are hard to deal with at the roadside; they're also prone to fade if you have to ride the brakes on long descents.
A lot of tourers come with cable discs. I've had my issues with these too, but sintered metal pads on cables are as heat-proof as you can get for those long heavy descents. A spare cable can be carried and there's never any need for bleeding. Short of trashing a caliper or lever most things can be fixed at the roadside, and any old bike shop could do something about a broken lever.
While bar-end shifters help separate the braking from the gears, some of us like brifters on tourers, and anyway, high-end groupsets only come with brifters.