I started out using game salad and to be honest I can't even compare them as game salad is way too limited and construct is so much more user friendly and powerful imho. I now have my own company making games for a living and construct is one of the biggest reasons that I achieved that. So I strongly recommend switching over.
1. There are tons of questions about this. But I usually just make a 16:9 game and use scale outer or scale inner depending on what type of game it is and with some simple tweaks your game will fit any screen perfectly
2. Exporting is one of constructs biggest flaws. Exporting to steam and web is very easy and mobile isn't that hard either if you know how to optimize your code. But exporting to consoles isn't really possible atm without third party porting. You can release UWP on xbone but it isn't optimal. My game that is waiting to be released through IDulb@xbox is still waiting for the missing features in the engine and it has been a year now. But I've heard great news about simple porting for construct games so I hope we'll see some real console releases soon.
3. Can't say much about that amount of physics objects as I haven't stressed tested it. But I had no problems working with physics with reasonable amounts of objects at least
4. Conctruct has letterbox scaling so making a pixellated game is no problem and you can scale it however you want remaining hard sharp edges as nearest neighbour is supported.
5. Making multiplayer games is easy when you get the hang of it. I've used photon to make some multiplayer projects and it is great. I thought multiplayer was too complicated before but photon made it simple.
6. You can do that easily with ajax for example. There are many different solutions and isn't hard at all.
7. I use third party plugins for push notifications and it works great, not sure the official plugins support it though.
8. C3 has visual list builders where you can make tables and stuff for your game so you should feel right at home with that.
This might sound like construct is all perfect which it is not, but it is improving all the time and is by far the easiest game engine on the market. I use unity and unreal but for all 2d projects construct is the fastest and easiest by far.
Anyway, that is my thoughts on it. I hope you'll end up happy whatever engine you choose. Good luck!