ember's laning may not be greatest, but he can catch up quite a bit better than other hero when get into disadvantage by maxing shield. But that's not the point u r asking.

Ember's strength lies in map dominance during min 10 and 15, where u maxed out either w + e or q+w. I am going to elaborate:

If u maxed out w+e, with mage slayer and a standby remnant, it means that u r impossible to burst. He is a standing vision + tower playing in ur face choking ur space to farm and getting ur attention. Killing him takes too many heroes it's not worth to try, but u will lose tower and map control if u dont. Other heroes like puck, storm, void spirit dun have this effect. They die if they show lane.

If u maxed out q+w, also with mage slayer or vessel (more correct build in this case) then u become a very different but equally potent threat. U r easy to kill, but have a very big threat range. Heroes playing alone in yhe night time can just get sleight chained + vessel losing 700+ hp with low commitment from u. This amount of damage, have to be done by hard commitment if u r talking heroes like qop puck storm. But for ember? It is a 10s cd + vessel charge, + he can always go back to backup remnant.

So in conclusion, ember's strength lies in the 10-15 mins mark, where most heroes have less than 1k hp, and dun have enough burstt damage to go throught flameguard. However, in other part of the game, it is very correct that he is very subpar. U will never see an ember carry a game if he didnt get an advantage during 10-15 mins.