They would probably be declared illegal by most tournament directors, because of the strong possibility that the tablet (or phone) might be running a powerful chess engine that could suggest moves.
Incidentally, “offering delay, increment, etc” can hardly be called “advanced”. No serious chess player should ever considering buying a clock that does not have both increment and delay.
Bill Smythe
In addition to such apps simply not being allowed, consider that the device owner would have to leave the device at the board throughout the game even if the owner leaves the board (for instance, to use the rest room).
Thanks for the thoughtful replies about these apps. I certainly agree that there are too many problems for them to be used at tournaments. Maybe there is a place for them at club or personal play where the cost of a full featured clock is beyond the budget of the players. It might help to introduce players to timed play at a cost of a few dollars.
Arthur Holmer