I have a Nexus 8 aluminium flatbar bike, a 27 speed Trek 520, and a couple of older mtbs.
Of the 4 bikes the Nexus 8 is the bike I enjoy riding least. In fact I haven't taken it out of the garage in about 3 months. It's headed to Fleabay, methinks.
It isn't the speed that is the issue with the Nexus 8. It's the range of ratios, or lack thereof.
The range of gear ratios more or less approximate the 12/34 sprockets and 42 tooth middle chainring on my 520. In addition to that range the 520 has a 30 tooth granny and a 52 tooth big ring. I don't spend much time on the 52 tooth, but I do need to change down to the granny on any serious hills.
In fairness, I should point out that I am 48yo, 5'10 and 265lbs. I am not extremely fit.
In my view the Nexus 8 is fine if you live in a relatively flat or mildly hilly city, and you are reasonably fit, or if you live in a really hilly city, and you are very fit. However, Sydney is a very hilly city, and that issue, combined with my weight and relative lack of fitness makes riding the Nexus 8 a pain for at least some portion of most rides.
That being said, on the flat I do wind the Nexus 8 up in top gear. For that reason I don't want to put a larger sprocket or smaller chainring on the bike. If I did that I would be spinning out.
As has been noted above, the Nexus 8 ratios are fairly uneven. This doesn't help, but it isn't the major issue with me. It is simply the lack of range.
I like the Nexus 8's lack of required maintenance and the ability to change gears at a standstill. However, in my case, they just don't compensate for the lack of ratios.
I played around with the idea of scrapping the flat bar bike and transplanting the Nexus hub to the 520, whilst keeping the triple chainrings. However this is a rather untested and uncommon setup and I did not want to run the risk of having two disabled bikes while I muck around and try to get it to work. I would be interested to hear of any multiple chainring/Nexus 8 success stories.
Some people love IGH. Heck, some people love SS/FG. But it's not for me.