Just completed a 2 hour flight in the latest AAU4 update of this aircraft (v2.0.5), using the S.55 1927 Jahu variant-livery (I’ve not checked out the other aircraft yet).
This flight was just a fun flight & I did not do a comprehensive check to look for any issues, but there is only one bug tagged savoia-marchetti-s55 for this aircraft in Bug Reporting Hub > Aircraft & Systems anyway.
I had a blast. It was supposed to be a short 15-20 minute flight to check out the new PG scenery in Cairns, Australia, so I started in the Barron river by the airport (YBCS):
This was a good test of the steering ability of the aircraft on water, as I needed to wend my way down to the coast for takoff. The trick here, since the engines are in-line - so you can’t steer using differential thrust like you can with the Albatross - is to keep up your speed up to provide airflow over the tail in order to steer:
Takeoff is a breeze, with fine control of the aircaft:
As with @Ramasurinen 's Albatross, the interior cockpit is highly detailed. I’ll not show screenshots - just check it out for yourself.
I’m not sure I’d ever want to be a passenger in this aircraft. It appears that you share the cabin with barrels of either extra fuel or oil:
No cabin service was provided, which probably is just as well, as there is no toilet either:
Maybe they used a tin bucket in the back with a curtain screen for privacy? 
I sure hope that some of the portal windows could be opened, otherwise the fumes inside must have been terrible! 
I was having such a good time that I decided to continue north to Daintree (YCBY), and found myself still in the air at Cape Tribulation…
Bloomfield River (YBMD)… Cooktown (YCKN)…
and Cape Flattery (YCFL), before deciding to call it a day at Lizard Island, ~ 200 flight-NM north of Cairns:
Dropping anchor in the bay by the small settlement on the Island.
In summary, a wonderful flight with no issues. If you enjoy flying older aircraft, I really recommend trying this aircaft out if you have not already done so.