I'd say it will be successful if it is capable of what tablets/smartphones can do (apps, media, internet). Otherwise i can't see its place on the market.
Apple and Android have years advantages in this. They are both multi million companies and invest ALOT money in iOS/Android to make it run that smoothly and get those functions.
Sorry but as an pure tablet nintendo can definitly not compete. Its not as easy to creat a great operating system. And google and apples have more people working on that software then nintendo has employers.
As a tablet it will fail. It will be slow, dont have many well optimized apps and much, much more.
Its more likely people will get controller addons for their existing ipads then get the switch imo.