A big problem with Ottomans is the timing.
Gunpowder, Engineering, Steel are late medieval/early renaissance techs, but there's three of them, so Gunpowder is difficult to get with great scientist to create a tech advantage. Bulbing gunpowder alone is not so significant, because of the vast amount of medieval pre-reqs, you could just go ahead and research it by the time you have the pre-reqs.
Meanwhile, Metallurgy/Rifling is mid/late Renaissance, and both high cost, and you can get a great scientist or two by this time, to create a substantial tech gap.
When I am rushing, I generally get Rifling 0-7 turns (quick) after gunpowder. This is not enough time to build Janissaries.
Therefore, Ottomans must have a slow/large rifle rush, with delayed rifling, in order to properly use their unique unit. They must also have a substantial amount of gold for upgrades.
Personally, I prefer a quick/small rifle rush, because they take less turns of preparation, and allow for more strategy flexibility.
Janissary rush alone without upgrading the Janissaries to riflemen is ill-advised, because 25 is more than 50% stronger than 16, and is only 2 techs away.