for the pros see above posts. For the cons (vs a putative 2 x 6 system say);
a) - 1x 11s has a worse wheel dish (weaker/heavier wheel)
b) - the chainlines are terrible.
c) - the 'cruising sprockets' are relatively small with a 1x system
d) - the 'chain security' argument is moot
e) - you cannot make a 'block shift' if you want to
a) means that the latest generation of MTBs now has offset back ends and even (I think) offset cranksets etc. If you had a ~130mm back end with (say) six sprockets on it, the wheel could be dishless with a wide flange spacing and the transmission could be set up symmetric with a low Q value.
b) means that when you are using the extreme gears, you are cross-chained about the same as (say) using sprocket #10 with the inside chainring of a 2x11 setup. This is inefficient and causes rapid wear. [claims that cross-chaining is 'worse' with more chainrings are entirely disingenuous; you don't use those gears, duh...
.]
c) suppose that you have a x4 gear range (say) and you set it up for 24" to 96". A ~70" gear means using a ~14T sprocket. This size of sprocket is both inefficient and will wear rapidly. If you skew the gear range towards the lower gears, you end up using smaller sprockets still to get that gear ratio.
e) means that some canny MTB racers have not gone with the 1x11 approach; they like being able to do a block shift on the chainrings.
So overall I'd suppose that it is somewhere between 'not for everyone' and 'a pretty poor arrangement' depending on how you look at it. The only reason a 1x system is lighter than a 2x system is because everyone takes it for granted that you must have heavy shifters and 10 or 11 sprockets at the back regardless 'because that is what modern bikes have'.
I think that (say) a 2x6 system (using modern materials) could have a stronger/lighter rear wheel, a better/symmetric Q, a better chainline, sprockets/chain that run better and last longer, would be more efficient, and would weigh about the same or perhaps even less. That is quite a lot of benefit in return for the penalty of a double-shift....?
-just my two-pence-worth, anyway....
cheers