Many of the components have to be beefed up to handle the stresses of repeated stops/starts and remain reliable. Further, some systems that ran off belts, for example A/C compressor & water pump, need to be changed over to be powered by an electrical motor. That also requires a beefed up battery to power these systems while the engine is stopped. In fact, if the battery is low, the start/stop may not activate. Then you need all the monitoring and management systems to keep it all coordinated.
While it is likely that the beefed up systems, like a starter, will last as long as a normal system component, it will cost more to replace the beefed up and possibly more complex component.
As it turns out, the cost in fuel to restart is insignificant once the car is warmed up.
And the reason for all this is to cut down on a bit of pollution and save a little gas. However, this may be gaming the system because the energy and pollution to produce all these beefed up components plus the energy costs to carry the extra weight of them around through the lifetime of a car might offset the benefit of stopping the engine for a bit. Plus it adds to the costs of recycling the bigger components at end of life. No one wants to take a hard look at that because it's an ugly truth. Manufacturers needed to cut pollution and get better gas mileage which government tracks but they added downside costs to other parts of car ownership that no one pays attention to. Just because it isn't monitored doesn't mean it is a net positive.
On top of all this, it simply costs more. If there was a positive accomplishment for the cost, maybe it would be worth it. But we may be paying more simply to play regulation games of no ultimate value.
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