My first thought is how rigid is the end of the table saw's table? A heavy router could warp the table slightly if it is the portable type.

nothing wrong with it if you're space challenged.  i've had a separate router table and a router in the wing of my table saw in the past.

It was the first thing I did with may table saw a long time ago now.  They salesman at the place I bought it from I remember now even said that's got to be the first thing you make.    It's got the same old paralock fence still
It's mounted on the fence wings, and I made a set of legs so no sag.    
The top is 1" mdf that I applied a laminate counter top surface to, and included a t-track fence slot.
I've upgraded the mounting plate a couple times now.  I have the Jessem mounting plate now and that should do it for me now.   woodpeckers makes a similar mounting plate, as does Incra.  Also got a dust collection hood under the table to help collect some chips and dust.

I built mine in to the outfeed portion of my home built table saw/work bench.

I have a  54” rip capacity on my saw. I filled the void with a 2” thick hard maple table top scrap I had laying around. Sagging is not an issue at all. I used a kreg router plate with kreg levelers.

I use 2 t tracks for fence adjustment with cam locks from rockler. I also have a miter Chanel t slot combo channel in front.

For a fence I use a kreg portable router table fence I had prior. You do not want to use the table saw fence. For one I didn’t want to loose my router settings every time I need to make a c pipe cuts with the saw.

I placed the router  6-8” further into the table than most router  tables to give more material support. It also shares my 3.5 ft deep outfeed table.  

No regrets. 90 percent of what I use mine for is a rounder or flush trim bit so I rarely use the fence.

I don't have a saw that supports this yet, but I plan to buy either a Grizzly or a SawStop at some point that has a router table. I do enough flush trim work (and roundover/chamfer work) that doesn't require a fence that it would be very convenient to have a permanently mounted setup. Cabinet work would still require the dedicated table and the guard.

I have a g1023rwlx that I added longer rails to. It was like $50 more for the rwlx to get the larger cast iron due to the router extension. But the router mount in the is crap. I’ve never even bothered to try an mount a router in it because of the design. That’s why I have the table in the extension.