while Harrison certainly was a popular desk in many large studios, i have never heard that Harrison was one of the best mixers AUDIO-wise. they were certainly favored by dirty rock producers(the music, not the producers). they were always considered thin and gritty, perfect for hard rock. i really do not think that the plugin is a good replacement for the actual hardware, but it is a pretty good sounding product. it's not that there is anything really WRONG with the hardware or the plugin, but i am still amazed that marketing has brought the Harrison name to the top of the industry. i can think of others that i would prefer. i think the opinion on the hardware differs depending on whether in Europe, the UK, or the USA...
these days the Harrison sound is considered very smooth and transparent(maybe that's why it was once thought to be thin), i don't know why the reputation was that it was gritty. afaik, the real advantage to the Harrison was it's build quality and it's EXCELLENT mic pres. none of this is in the emulation.
i'd say that this is really UAD's(and others who license names) biggest advantage. the fact that they can claim that their emulations are more properly made, since they are allowed to use famous trademarks. the reality is that emulations sound NOTHING like the original, if you want to be pedantic about it, but that doesn't means that they are not useful, or that they do not sound GREAT.