Opposed twins (180 degrees opposite) with magneto ignition timed to the flywheel are able to utilize a single magneto to fire both cylinders, exploiting the "wasted spark" characteristic.
V twins require the use of a magneto for each cylinder because the 2 cylinders are at unequal divisions of 360 degrees.
Having 2 independent magnetos to kill when you want to stop the sparking requires a properly constructed diode pair as part of the kill wire circuitry, otherwise each magneto would need its own separate kill switch scheme in order to stop both mags from sparking.
Other ways to provide ignition for V twins would include a flywheel sensor timing an electronic ignition system to deliver a spark to each cylinder at the proper time, and that is more complex than just using 2 mags and some diodes.
The advent of OHV primarily came about because of the need for improved efficiency of fuel use. That has ramifications related to both power output per pound of engine weight and fuel economy/EPA emissions standards.
The old school F head engines are just too thirsty and too dirty to compete with the modern OHV athletes.
The V configuration won out over opposed twins because the V shape is just enough more compact in 3 dimensional space to be attractive to the cosmetic design engineers because it permits a twin cylinder engine to fit into a narrower (or shorter) space than the same displacement opp twin.
Fitting into a narrower/shorter space means a more compact hood requiring less of everything to conceal the more compact engine.
While an overhead valve, opposed twin (or 4 cylinder) is certainly viable (just look at all the examples that were built for VW, Subaru, and motorcycles down through the ages), to develop an OHV for outdoor power venues would probably have been a risk that the OEMs were just not willing to gamble with due to the increased width, or length (depending on how the engine was placed) cause by the added valve covers.
In that respect, it truly came down to cosmetic "battle of the bands" in the quest for market share.
Single cylinder engines are somewhat simple in the numbers game because there is only 1 of everything, but that 1 of everything has to do a lot of work whereas the workload is divided between 2 cylinders in a twin.
Twins tend to be a little smoother operating engines than one lungers because of the division of power and load across 2 cylinders.
My big objection is the current (or at least "in recent times") strategy of wringing more power out of single cylinder designs that actually were intended for less output power.
Yes, the OEMs can "force" a design to make more power than was originally intended, and it will seem to be a good idea for a while. But just like a hot rodder takes an underpowered engine and starts souping it up, the longevity and durability will suffer, often with a catastrophic failure (con rod through block).

Well-phrased answer Mownie!

I have always enjoyed reading your posts here. You not only know yer stuff, you write it well too.

Sometimes I feel as though my mind is racing like an engine running against the governor (and I don't mean that in a good way).
It is the wonder that anything comes out in coherent form at all. :^)