Hearing the issue you described above is a first for us since this saddle made it to market, and it never came up in the 2 year testing process......and in testing we had folks XC racing on it, race the Tour Divide the last 2 years, multiple Colorado Trail Race efforts, multiple solo 24 Hr efforts in Europe, and countless marathons and stage races.
As others have mentioned, saddles are a different subject for each rider. So while the saddle is good for you for 4 hours, it may not be for 20 hours. I wish I could explain that, but I can't.
As an endurance racer myself, the body changes when in the saddle that long; swelling, inflammations, etc. I typically ride the SM3-S for all racing; marathons, 100-milers, etc. When I used the SM3 for my CTR attempt a few years back, in training the saddle began to feel narrow after 10+ hour training days. I ended up jumping up to the SM3-M and it made a huge difference in comfort and pain over the days I was on trail. To this day, I now ride the SM3-M. In the Ergon saddle measurement tool I measure up on the high end of the S and low end of the M. When riders or retailers come to me saying they are in between sizes, I always suggest they aim towards the larger size.
As far as sizing, note that our saddles are measured off the the sit bone spots in the saddle shell, not the overall width of the saddle shell. For example, if your sitbones are spaced at 155mm that would put you on our SM3-Large, which is for sitbones spaced 130mm-150+mm
The sitbone/rear area of the SM3 saddle is "relatively" flat. You should not feel any ridges while riding the saddle. I have had some riders mention this to me as I have traveled to events and shops across the USA. A simple fore/aft adjust cured it. In general, our saddles when installed are further forward on the saddle rails that what most are used to with other brands.
Shoot over any questions. Will pop back onto MTBR as time allows.
Jeff K
Ergon USA