Wireless chargers do generate waste heat, but a smart device will signal the charger to stop charging, so it's not like leaving a pot on the stove and burning up the contents.
That said, depending on usage habits, leaving a phone on the charger, and resultingly keeping the battery at a fully charged level for long periods is also not ideal, regardless of whether it's wired or wireless charger.
In general, Li batteries are happiest when in a partial state of charge, as well as being stored that way. The actual numbers may vary, depending on who is asked, but keep in mind Li battery powered devices leave the factory and sit in inventory partially charged on purpose, and new transport regulations will reinforce that from a safety standpoint by dictating that they ship at a maximum of ~30% charge.
Of course, in practical use, few are going to charge their devices to a maximum of 80% and then recharge at 40%, sacrificing capacity for the sake of optimal durability.
But, charging your phone to 100%, and then using it until it drops to 20% at the end of a day (or whatever period), is not the same thing as leaving it on a charger most of the time and keeping it topped off at 100%. Laptops that serve stationary duty connected to their mains adapters for much of their lives do not exhibit great battery life when asked to actually perform mobile duty, despite having few charge cycles.
Heat is also detrimental to battery durability, but again, they heat up regardless of how they're charged. Whether the additional waste heat generated by the inductive charging process is unnecessarily harmful, and by how much, will depend on the specifics of the setup.
Batteries manufacturers have their own acceptable parameters, as well as Apple engineers. One hopes that they all work together to provide the best experience to the widest number of use cases, but it's questionable if empirical testing has taken place to back up the expected projections under every circumstance.
These are not exact sciences, and there are debates involved even between experts, so there are a lot of generalities and it's hard to answer specifically.