May be as you say, nevertheless there is a limit of speed to be achieved via this method, which will always below a lot below the speed when using a dedicated keylayout or even dead keys, if you are familiar with using them (my wife says dead keys are horrible, and could not convince her to switch to using them).
Picking chars, as in iOS, is indeed a simple way to write a very limited amount of data, as the example with names with diacriticals in Address Book. That is why iOS is totally inappropriate for writing longer texts if diacriticals are needed. To say nothing that the virtual keyboard on an iPad is something difficult to use for longer texts. Of course, I am subjective, but I think that using these alternative methods will always be below the required speed for normal writing.
Summing up: if someone is accustomed to the U.S. keyboard, and must write in French, then Canadian French is the best solution, and also the fastest, especially when long texts are in view.
For short texts, e.g. names or isolated words, any keylayout allowing dead keys is simple solution.
In iOS, the only current solution is either a dedicated keylayout (dunno whether French is there) or, using char picker, only for isolated words e.g. names in the AB.
Of course, experimented users may use UKELELE and create their custom keylayouts, this is simple enough when adding several chars only, any user may put them where she/he wishes.