Tankless heaters
I will assume you understand the benefits. No huge storage tank is constantly bleeding off heat, which then has to be reheated again and again when not used. Also less space taken up, energy savings, ease of replacement (it’s just hung on the wall) and low cost to purchase.
Now to the negatives. Smaller tubes that need to be delimed every year or so. Properly installed with shutoffs and a small acid proof pump ($150.00) and cleaning vinegar or professional deliming solution and a half hour takes care of most problems. Another problem is installers who use the wrong brand (home use tankless) and charge outrageous prices to plumb in the heaters.
What most people who view them negativity, is the hard water (mineral enriched) will lay down in both traditional heaters and tankless heaters. Larger pipes just means it takes longer to clog. Both types need to delimed on a regular basis.
The line buildup inside the heater acts as an insulation which stops reduces the transfer of heat from the flame to the water. Over time standard water heaters will require more and more firing time to maintain temperature levels but this loss of efficiency is not as readily noticed in tanks with larger piping found in traditional heaters.
Your best advise is use Navien heaters that have models with circulation pumps included and feature stainless steel in their heat exchangers.