1. Playing zelda 3 hours straight in handheld mode is short, but I would argue that in a situation outside the home where 3 hour sessions aren't a natural occurrence, the sleep mode will allow it to survive inbetween breaks.
2. The handheld itself is actually barely larger than a mini tablet with the joycons detatch, so if you feel you need to bring both with you, you can just fit them into something like a bag or jacket pocket.
3. Handheld mode is actually the state being barely more powerful than the Wii U, as a home console it's more further away from it.
4. This is more anecdotal, but disregarding the joycon desync situation, which was acknowledged as an inherent problem with low power bluetooth devices and will have a fix in the future, the joycons are actually pretty good controllers in of themselves and the ability to just be able to have your arms free from one place to play a game is invigorating.
5. I will concede with the online BECAUSE we still don't know much about it. Online play is serviceable, but without much more info on how the paid service will benefit that beyond "monthly rentals" and the phone app, but without a lot more info about this "will it be confirmed to include cloud saving, does the phone app sync to the Switch itself so you can hear both game audio and gamechat, any plans to provide more info on the monthly game rewards, etc) That's all on Nintendo, it's dumb that we still don't fully know.