I would say Nagamaki is better than a Katana or rather say a Uchigatana or perhaps a Tachi.

A Nagamaki has:

More reach: due to a longer hilt that is as long as the blade and the entire sword can be as long as the user

Versatility: a large portion is a hilt so you can do various grips

Ease of applying power: Due to leverage and raw mass

While still being able to be unsheathed like a Uchigatana since the blade length isn't extremely long. The disadvantage it has compared to a Odachi is that it has less blade portion.

That being said Nagamaki are harder to craft due to longer length and the hilt also needs to be durable to high quality materials are required.

But at the end of the day what you are trained in is the best for you of course unless that specific item was low quality.