Cannons are more than ever reliably ways to nuke a dragon, be glad the WE dragon gets a 3+ ward, my beloved chaos dragon doesn't even get that

.
S4 breath weapons are nasty as all hell, they'll wipe out most of a T3 unit, don't underestimate them, yes once a game, but now they work in combat AND shooting.
That said, monsters were very difficult to use even in 7th edition, now it's even harder, but can be very deadly never the less.
Any solid objects stop a cannonball now, so if you're flush against a building, the cannon can't get you, even if it can see your head/wing etc. Other monsters also stop the cannon if they survive(for me it's a warshrine because it's survivable, but other things work just as well, depending, thinking hydra here).
The biggest asset from a dragon is the model is BIG and SCARY, people will ruin their plan, panicking about knocking the dragon out, it's other asset is being able to strike anywhere on the battlefield, for example versus lizardmen you can fly overhead and knock the slaan out early on in the game.
That said, monsterous mounts have never been very newbie friendly (in spite of being very attractive to a new player) so be careful if you want one and be ready to watch it die....a lot.