I think the biggest issues out there now are resource management with hwa, knowing the workarounds for "features", and the less than obvious issues DirectX 9 brings.
A platformer, shouldn't be a big deal, but 2.5d on the other hand....
Well that's more of a complexity issue rather than ability. There is actually quite a lot of built in functionality for that, parallax layers, z height, z index, etc.
I would suggest figuring out what would be the hardest thing to implement, and testing that out first. That should give you a hint as to whether Construct is for you.
Then the forum is always pretty helpful for specific stuff that might evade you.