Personally I don't think that Wii U is based on "Broadway" since Nintendo's own site says the Wii U CPU is a IBM PowerⓇ-based multi-core processor and the original Wii was a PowerPC CPU (code-named "Broadway") jointly developed with and manufactured by IBM.
http://www.nintendo.com/wiiu
http://www.nintendo.com/wii/what-is-wii/#/tech-specs
IBM Power and PowerPC CPU's are similar architecture but still a tad different....
From IBM's press release on the CPU for Wii U:
"Built on the open, scalable Power Architecture base, IBM custom processors exploit the performance and power advantages of proven silicon-on-insulator (SOI) technology. The inherent advantages of the technology make it a superior choice for performance-driven applications that demand exceptional, power-efficient processing capability from entertainment consoles to supercomputers."
http://www-03.ibm.com/press/us/en/pressrelease/34683.wss
I'm not saying that developers aren't having issues with the CPU, but they may just need to learn it a bit before getting the full potential out of it. Porting is not simply going to be cutting and pasting Xbox 360 code on to the Wii U dev kit and expecting magic to happen......