My observations from practice today compared with testing is if we consider last year, it looked like Merc were miles ahead and come Australia and the opening races that clearly wasn't the case - Hamilton pulled out a great lap in qualifying but Kimi was pretty closer to him on race pace. Since Barcelona got resurfaced last year it has had an unusual amount of grip, plus conditions in winter testing are also always largely unrepresentative. Merc last year benefited from that extra grip with a car that was inconsistent in the opening rounds and overheated its tyres. We now have Ferrari ironically in the exact same boat potentially - the car worked well in colder conditions on a high grip track, but is not working in hotter more representative conditions on a low grip track.
I am unsure of the issue with Merc looking stiff - McLaren for years ran with very stiff suspension when they used to win races and be in championship contention. Maybe it's just how Merc have been able to get this particular package into the envelope and crucially it isn't losing them significant performance - the car looks twitchy but also alive and responsive. It goes to the saying that just because a car is well balanced and planted, doesn't mean it will be fastest. I remember Alonso used to say McLaren had a really well balanced car, but it was just slow! I'm not saying Ferrari are slow but maybe the conditions in Barcelona have flattered the car a little and their extreme front-wing design philosophy was perfect for those conditions but may not be as well suited to traditional tracks. Maybe Merc's car always had more pace it was just difficult to drive, but now they've tamed it, it's let the beast loose.
I do recall James Alison saying a big focus of this car was on tyres and ensuring they worked them better than in previous seasons as e.g. Red Bull was far better on tyres in 2018. If they have succeeded in this, then I worry for the competition! Aside to Merc having good long runs, I was also impressed how on both short runs on the softest tyre in FP1 and FP2 Hamilton was able to put in better sectors after his first hot lap, so they're keeping the tyres alive well. In-fact Hamilton went slower in sector 3 on the soft than he did on the medium, so there's more potential there.
A final point I don't really understand is what Christian Horner has said about Merc being very stiff and difficult to drive and questioning whether that's sustainable over a race distance. Unless he thinks the car will literally fall apart, which would be bizarre, then I don't see what he's referring too - they clearly are dealing with tyres OK and their drivers aren't making mistakes (aside to Bottas' fun in the grass) so it cannot be that difficult to drive. This almost seems like a slightly childish remark made because they are annoyed that Merc are so quick and are making a stiff suspension work where normally it would not on a bumpy track.
We well know from the last few years that the best way to make Merc even stronger is to put them in a mini-crisis. Every time they had one bad race they would come back stronger and often be unbeatable there after. Well testing was the crisis, and now we see the result. The jury is out on how much of this is for show!