Yeah, if you don't have the panel space, it's probably not worth it since panel upgrades are expensive.
Really a Gas tankless is better than electric. Gas is cheap and more efficient right now since the technology for electric tankless isn't quite there yet to compete with gas. That being said, since I don't have gas at my house, if I ever had the money to have my panel upgraded, (which I should do regardless of a water heater), i'd go to electric tankless.