It's a question of what you want in your army composition and there will always be tradeoffs. If you want to have 2 units where one is solely AT and the other can be flexible AT/AI then that's fine. But 2 E8s, while more expensive also have the benefit of being generalists and more durable than the M4A3.
You don't ever have to worry about the Jackson getting caught by AT Infantry or the M4A3 getting outmuscled by other tanks (or being forced to use them together all the time). Easy 8 will almost always be the outmuscler and that's not nothing. It's not going to be great AI and it's not going to have Jacksons's Penetration ceiling but those are choices you willingly make and factor into your army composition. Those tradeoffs by themselves don't mean the E8 is trash. More simply put, don't build the E8 and expect it to perform like a Comet because it wont.
I see what your saying but they are generalist in the sense that if rng rolls your way you will do some AI damage. It will not punish blobbing like the regular Sherman HE will and to get the improved AT performance you have to invest heavily with more fuel, muni and loss of mobility.
So I end up punishing myself instead of my opponent until the E8 hits Vet2 where it finally starts to work as intended.
The outmuscling is only in a vacuum 1v1 vs OST tier 3. Looking at cost, the OST player could just make a Stug with the 2nd one not far away vs 1 E8. Yes, the E8 will win 1v1 but against 2 its losing no questions asked which is to be expected. Making the Stugs is a minor inconvenience for most players and does not require any specific doctrine to make. For OKW, only the P4J is getting outmuscled from the STOCK choices available.
Now looking at Army composition an OST player can be expected to have an AT gun which will reliably push back the E8, OKW can spam raks however the E8 has a decent chance of deflecting. This allows the Axis player to stick to their strategy which means my choice only disadvantaged me. I did not force them to hold out for a Panther which would give me at least a minute or so to create momentum.
The reason most have such high expectations of the tank is due to the relative weakness of the entire company. So like some have said the tank needs to carry the doctrine or the rest of Rifle company needs to be reworked.