Probably nothing but its a brand I havent come across. Have you ridden one?
I found myself having to ride an unfamilair brand in France when my Dawes Galaxy fractured a rear drop out. As the Galaxy was close to needing an extensive refurb after the tour, I turned up at
Decathlon in Orleans and found a B'Twin Triban 7 complete with rear rack, 24 gears, hub dynamo lighting all for £400 all I had to do was buy pedals and swap over saddles. The bike looked as ugly as sin but I could have spent a day or so trying to get the drop out brazed with no guarantee of the fix or curtailing the tour with additional expense of getting back home a week earlier than on the booked services.
The spec of the bike indicated that it was up to the job and it proved to be the case. It was as comfortable and as fast as my Dawes Galaxy. I used it for touring and audaxing and it's now owned by a pal who came on tour with me in Germany Austria and Hungary this summer.
There's nothing in the spec to suggest that the Jamis won't deliver. If you want to consider something in the same price range look atthe Decathlon Riverside 7. Everything that my Triban, now out of production, was and lots more.
I'd not bat an eyelid at purchasing the Jamis if I was in the market for a £600 tourer.