True.Theoretically, at level 10 if you blew all uses in the first fight you would get 5 more uses due to 5 additional fights.
And at 15, battlemaster could 5 more by blowing all his dice in the first battle.
True. And there's plenty of other differences as well. Like trip could fail, and your allies could get advantage or disadvantage (if ranged). As well as samurai use their bonus action, and get's THP.Battle Master dice uses vs. Samurai attacks with advantage isn't necessarily apples-to-apples.
But overall I think it's a reasonable comparison.
The bigger point is that battlemasters are front loaded, samurai are back loaded.