Tankless are not necessarily more efficient.
I have never seen a tankless unit that wasn't more efficient. Storing a bunch of hot water is pretty damned inefficient no matter what you do.
And if they break, they can be expensive as fuck to fix.
Depends on the unit. A tankless system might require a replacement electronics board if it's a newer system
Not to mention the inevitable upgrade to the electrical system if it is electric.
Very few companies make tankless electric heaters- and when they do they are usually smaller units. Obviously if you are going to have to spend a fortune to upgrade your electrical system- then a tankless unit doesn't make sense.
Gas ones (Rheem) work pretty well, and are a lot more efficient that electric, obviously.
I have several neighbors with Rheem units and I have a Rinnai that I've had for almost 20 years now without a single problem- and it's a LOT more efficient than that tank heater it replaced.
A lot of tankless system are not very reliable. Not sure where you are you getting your info on reliability from.
Not sure where you're getting your information from either but tankless units can be very simple and reliable- or they can be complex and less reliable. I still have an Elm (later Bosch) Aquastar in that has been in use in my family's summer home since it was installed in 1984. It literally only has a few parts- and they are all easily replaced if there is a problem (In 33 years we've replaced the pilot temperature probe and the flow switch). Meanwhile tank units often corrode through and the first sign you get of a problem is a basement full of water.
Rheem makes a plastic lined water heater that is essentially maintenance free and probably will never die.
Current tank water heaters are all lined- otherwise you'd get rusty water. The Rheem Marathon water heaters, meanwhile, are not plastic lined- they are entirely plastic. The problem is- that only works if you use electricity to heat your water which is a non-starter in a lot of areas.
Tank water heaters fail because the tank itself is made of steel- and even with a lining- it will rust.
Tankless water heaters are made of copper and other materials that aren't going to rust out the way a tank water heater will.