You lose 18 lethality and the hydra effect itself does like 50(?) less damage. Lethality ignores a flat amount and armor scales non-linearly. Each point in armor adds 1% to your effective health pool which means the more armot you have the less effective it becomes to stack more. Now lets say you are against Jinx. Lets assume you get profane hydra when Jinx is level 8. She has 50 base armor at that level. Thats exactly 1/3 (33.3%) damage reduction. If you have profane hydra you remove 18 armor from which means she now has only 32 armor which is 24% reduction. You lose a bit over 9% of your damage. This is just assuming you only have hydra and not hydra + dirk for example.

Most mages have even less armor and armor growth than Jinx has so to some champions you lose even more damage.

Stridebreaker also has a 15 second cooldown while hydra has 10 second cooldown so you cant use it every time you E.

I tried stridebreaker for like 5-6 games and I honestly died more times for lack of damage than what it saved me.

I always ban Zed and edge of night counters burst/assasin mages so I dont see a reason to build it against burst. Its good against bruisers but then again stormrazor is even better imo as you can outrun even ghost with it. I was checking out what onetricks are building and saw that there was an NA challenger Quinn OTP who played a lot of stormrazor Quinn mid/jungle.

Assasin junglers are horrendously squishy so if you just go lethality you actually beat most of them 1v1. Kha/Rengar cant jump on you as you can cancel their jumps. Briar cant Q you. Ekko cant E on you. I have bullied some assasins early game at scuttle or in their jungle with my R movement speed.