Are any structural concerns with making the table this long?
No. Even with a top much thinner than you're going to end up with you can have a >100" span.
Would the table need to be reinforced underneath with hidden brackets?
Not at all, the interlocking box joint is very strong (lots of surface area for the glue) and the table will be naturally stiff.
How much weight should it be able to hold?
You could try using The Sagulator to calculate this, but it won't be easy since this isn't any material that it includes (because of the way the boards are oriented). Let's just say a lot.
This design orients the boards edge-up, which is using the wood at its maximum strength as regards resistance to sagging. Couple that with them being glued together what you're making here is in effect a laminated beam, so its strength is immense. I bet a heavy person could stand right in the centre and it wouldn't bow enough to notice.
There is a weight issue that you might not have thought of and that's the weight of the table itself. Obviously you like the chunky style but do be aware scaled up to table size this makes for a great mass of solid wood and it'll weight quite a bit and may be difficult to move around even just made from 2x material. Made from hardwood it would be a two-man or three-man job in a lot of cases!