Taking a rational and comprehensive view of the advantages and disadvantages of instantaneous water heaters

Tankless Water Heater Pros
Endless Hot Water: Tankless heaters provide a continuous supply of hot water, which is ideal for large families or those who frequently use multiple hot water fixtures simultaneously. "Biggest pro is space savings and endless hot water."
Space Savings: These units are compact and can even be mounted on an exterior wall, freeing up valuable space in your home. "It will obviously take up less space, you could even consider mounting it to the outside wall and doing some minor waterline rerouting."
Energy Efficiency: Tankless water heaters are generally more efficient than traditional tank heaters because they only heat water on demand, reducing standby losses. "People love gas tankless. They do not like electric tankless."
Tankless Water Heater Cons
High Initial Cost and Installation: The upfront cost of purchasing and installing a tankless water heater can be significantly higher than a traditional tank unit. "An EASY install costs about 5k, most are closer to 8k."
Maintenance and Descaling: Tankless heaters require annual descaling to remove mineral buildup, especially in areas with hard water, which can add to the long-term costs and hassle. "You have to descale the heat exchanger once a year at least which is another cost added."
Electrical and Gas Line Upgrades: Installing a tankless water heater often necessitates upgrading your electrical or gas lines to accommodate the higher demand, which can be an additional expense. "You often need to upsize the gasline to accommodate the hungry unit."
Delayed Hot Water: It can take some time for hot water to reach the faucet, especially in homes with long plumbing runs, unless a recirculation loop is installed. "Do you mind if it takes a bit for hot water to come out of the faucet."
Performance in Cold Climates: In colder climates, tankless heaters may not be able to provide the desired temperature rise, resulting in lower flow rates of hot water. "Or live in a cold climate where a 11gpm unit actually makes 3.5-4 gpm because of the temperature rise."
Electric Tankless Water Heaters
High Energy Consumption: Electric tankless water heaters are particularly criticized for their high energy consumption and the need for significant electrical upgrades. "Electric tankless water heaters are not efficient and have very high energy requirements."
Limited Effectiveness: Many redditors suggest that electric tankless heaters are only suitable for low-usage applications, such as a single sink. "Electric ones aren't effective for more than a handwashing sink."
Alternatives
Heat Pump Water Heaters: These are recommended for their high efficiency and lower operating costs, especially in areas with moderate climates. "Get an 80 gallon heat pump water heater. 80 gallons is a lot of hot water and it is by far the lowest cost way to heat water."
Recirculating Pumps: Adding a recirculating pump to a standard tank water heater can provide nearly instant hot water without the high cost and complexity of a tankless system. "Get a recirculating pump, you‘ll thank me later."
Conclusion
While tankless water heaters offer some attractive benefits like endless hot water and space savings, they come with significant drawbacks including high initial costs, maintenance requirements, and potential installation challenges. For many homeowners, especially those with moderate hot water needs or living in cold climates, a traditional tank heater with a recirculating pump or a heat pump water heater might be a more practical and cost-effective solution.