Adding electronics to a bicycle shifter seems like an incredibly bad idea. Just because you can doesn’t mean you should.
My girlfriend has the top of the like mechanical GRX equivalent, and while it's also a faster shifter, the Di2 is just a bit faster. Both are equally reliable, but hers will ultimately require more maintenance (cable tension, replacement parts) and is more sensitive to a bent hanger.
The Di2 nterface is dead simple: A single cable CAN bus for data. This also carries power for all the attached parts to the bus. The other benefit I love is sequential shifting. The onboard computer moves the front ring for me automatically and downshifts the back when I hit the threshold point. This ensures optimal power transfer by avoiding oblique angles with the chainline.
All in all, it was a great investment and I very much like it.
CAN bus is 2 wires and seems an overkill, a LIN bus (single wire) with power seems more suitable.
Modern Shimano drivetrains with Hyperglide can be shifted under "quite a bit" of torque, but I wouldn't say a 1x12 can be shifted under "full gas". I've yet to break a chain on my 2x11 GRX even at full stupid. My 2x11 even survived Unbound Gravel this gear with the _absolutely_ mudfest of riding conditions in the second half of the 100mi race. I have broke a chain on my 1x12 under full gas though; is an XT Cassette, XT Chain, and XTR derailleur and a plain-ol mechanical shift cable system.
All that being said, you absolutely should prep your gears at the same time you're prepping your body position. GRX/XTR and to a lesser extent Di2 allow you to delay this to the last second.
The brakes aren't electronic, only the gears.
Electronic shifting has been a huge milestone in cycling. It's better than traditional cable shifting in almost every way.
For me, it is about lever feel. The up and down shifts are too close together. I've had this happen before, where I meant to shift down. Didn't work, so I press the button again, it is yet worse. Oh, I shifted the wrong direction, but now it is quite steep and I am quite misgeared. And shifting under load is bad for the system even if a motor drives the mech. Now i make that mistake less often, but I am still generally displeased about Di2. (Why am I on it then? My frame won't accept mechanical shift cables, and SRAM was out of stock. And I really wanted that bike.)
Also, nobody really needs 12 speeds. 1x13, you can almost cover the same range as 2x10 but keep the same ratio between shifts (there are a lot of redundancies in 2x10). And then no more front shifting. 1x14 and you are there. Wake me up when that happens. Yes, Campy Ekar, I know.
You need an absurdly larger climbing gear on MTB (51-52 tooth) for climbing and wheelie-ing the front off the ground while on inclines. You need a relatively small gear for some speed on downhills and to load the suspension.
And because the terrain changes rapidly, you need to fly between the combinations quickly and any point on the cassette. Being able to dump a few gears and quickly pull them back in a few seconds later is a trick you can't pull off with a 2x setup. It's a party trick a 1x12 does in practice and still gives you reasonable jumps between gears.
A 1x12 is also a pound or two lighter than a 2x setup. It also is _far_ more reliable... Dropping a chain with a big-little chainring is unheard of and this can be done without a chain guide or bash guard. Losing these two components is another weight savings... You also can't do big-litle chainrings with a 2x setup, because it's nearly impossible to derail the chain.
Finally, and maybe most importantly, a 1x12 free up cockpit space for a dropper lever on the left side or a suspension lockout lever (if you're into that).
I recently bought a 105 level road bike with wide tire clearances and stuck the largest gravel tire I could fit on there and use that in place of my MTB for riding the gravel paths around town. It's nice to be going fast again.
MTB manufacturers listen up - make frame clearances support larger than a 39 tooth ring up front. People like me will thank you.
Also - back in the 3 ring / 10 speed days I was riding full XT and chain drops were a similar non-issue then too.
Now knowing that 700c roadie wheels and 29" MTB wheels are the same circumference ... riding a roadie on gravel with a 48 up front and a 11 out back is comparatively fucking fast!!! Im not going to take any "real trails" on the thing but as the gravel path around the lake goes - fitness rides etc - it hauls ass like my old MTBs that had the similar gearing. I really miss it to be honest.
I was doing a group cycling tour with 3 friends, one of whom had a Di2 GRX shifter. He pinched the Di2 cable in the rear axle when reassembling his bike and shorted the whole system. Needed to replace everything. That ended the trip for him, had to go back into town and get it fixed.
The system might provide a better shifting experience, but from a durability and field-repair standpoint I think mechanical shifting still has the edge.