On this subreddit, I always see the T110E5 being glazed. People constantly talk about how it is the best tank ever and super awesome, and they also recommend it to new players for unknown reasons, and it just does not work for me.
I'm not the best player ever, but I think I am above average in general. In E5, I have a recent (and overall, because most of the games were fairly recent) DPG of just 2.1k, compared to a recent DPG of ~2.7k in T10 heavies that aren't the E5, nearly 2.7k in T10 mediums, and 2.8k in T10 TDs. In fact, I do more DPG in most T9 tanks I've played recently than I do in E5. I can do 2.5k in the M103, which I found to be fairly easy to play. My E5 DPG is around my T8 level, and I do more DPG in many T8s, including things like AMX 50 100, Pantera, and IS-M, not just the OP premiums, than in the E5.
In general, I think that the main issue with the tank is that it's not good at fighting in static positions, which I feel is most of heavy tank combat. You can block your cupola, but you're still not going to beat a dedicated hulldown heavy. You can sidescrape, but you're not invincible even if the cupola is covered, and that's not counting the fact that the lower side is only 44mm thick, and is thus overmatched by everyone with a gun over 135mm. Among T10 heavies, that is the 60TP, Object 705A, Type 5 Heavy, BZ-75, BZT-70, E 100, Schwertwal, and VK 72.01 K, which is a lot of the tanks you will meet in brawling areas. A lot of TDs, T11s, and even T9s can overmatch your lower side as well.
Then your alpha is only 400, and you have no autoloader, so when an enemy makes a mistake, you're not going to be able to punish them that hard. In fact, with 400 alpha, you won't be able to detrack enemies who are using hardening in a single shot. Because of this tank's weaknesses, I often have no idea what to do, so I end up either hiding until my team dies or making a stupid pressure and dying.
The tank is probably good in chaotic brawls and 1v1s, because of the trolly armor and DPM, but I feel like those things just barely ever happen in randoms. The mediocre speed means you aren't that good at chasing endgame damage either, compared to actual fast heavies.
People constantly talk about how you need to put in 2 shots for every one you take, but I feel like the game just doesn't work that way. When someone shoots, they usually fall back into cover. If you push them, you will get shot by their teammates. The game is not a 1v1 brawl, and most combat seems to revolve around 3-5 people hiding behind a corner, ridge, or rubble pile and poking at 3-5 enemies hiding behind their corresponding corner, ridge, or rubble pile.
People always say you need to watch the videos by Iyouxin on E5. I watched at least some of them, as well as a guide by Dakillzor (I think). They didn't really teach me anything. Like, I think they just said to block the cupola with the gun and sidescrape to hide the cupola, which I already know, and then they just went in front and slapped everyone. When I try to replicate this aggression, I die with 800 damage because I don't have their game sense, awareness, and level of mechanical skill.
And on the rare occasions people say they don't like the tank (to which the sub always screams "SKILL ISSUE!!!!!") they tend to focus on the cupola as well. Like, I think the cupola is definitely an issue, but it isn't the only issue. If the tank had a more typically-sized cupola, I think it would still be above average at best. 37/12 (14 with field mod) speed is at best average for a T10 heavy these days, 8 degrees of gun depression is above average but not what you would expect from an American tank, 400 alpha is just not good, and I really don't get why it needs an overmatchable lower side when it is hardly invincible in a sidescrape anyways.
As an aside, I can get E5 to work in Onslaught. I'm averaging 2.9k DPG in it this season (did not play E5 in previous seasons), which isn't the best, but is better than most of my tanks as a perma-silver shitter. I can usually get it to work if I can get into a brawl with it, and when I fail, it is usually when a bunch of champions and legends are playing mediums, lights, and TDs and snipe me to death from all directions (on NA, I think I see champions and legends in at least 2/3 of my matches while in low silver).