Do you only ever play with the base game? Only play with 4 heroes? Usually four investigators but I have all the expansions, but usually play with just one plus a herald. I think that's the hardest mode.
"I will definitely have to disagree, every expansion but 1 adds gate bursts, adding all the expansions in significantly increases chances of a gate burst, and increases ways in which the terror level can rise..."
I'd agree that 4 players isn't the hardest mode over all. But we only count sealings and closings for victory. I think 4 players is a reasonable level for that goal. As you drop to fewer players, it becomes harder to next to impossible to seal. We don't find fighting the Old One to be as fun. So I guess that's a house rule. So we've restricted the victory conditions.
However limiting the game to one expansion and herald gives you the full flavor of that expansion. When we play with the small box expansions we don't play the tournament mode. KiY is really tough when played that way with its herald. Black Goat Herald is tough for us right now. CotDP is so, so even with its herald. Ghroth adds two doom tokens right from the top, but of course he'd do that with playing all the expansions too. Tulzscha is highly variable. So we increased the ferocity of the small box expansions.
As to your above comments, terror level doesn't really add to difficulty does it? Gate bursts have now become a generic element of the game as have cards that raise terror. They are fixes in the earlier designs. Gate bursts do make the game harder (and longer) and I had two of them in my last game with AH and DH. That's plenty! A quicker rising terror level only makes point scoring lower. At least I never had a game where the Old One came out because the Terror Level reached 11. Is that common or even happened in your games?
"(e.g., Dunwich Horror tokens), furthermore investigators have to spread out much more, so it's tougher to keep the monsters in arkham in check."
?? Half the time we play with one big expansion board. I do agree that Dunwich requires a constable or two depending on the number of players, but try ignoring Kingsport and see how much harm it does. It's next to nothing. It will probably be 10 or 12 or more turns before a rift even comes out and then only 1/6th of the time after that will it add a doom token and a game only lasts about 20 turns so with really bad dice you might get two doom tokens and a 4 extra monsters. You can get that with Ghroth right from the getgo.
Its really the heralds that make the game harder, IMO. Black Goat expansion does little by itself. Without the heralds the expansions are pretty much the same game over and over. When we play the small box expansions, we play the touring version, so that we really get the flavor of the expansion. With a Third Act coming every 5th or 6th card, that alone will add 2 doom tokens or else the game will end early. Even with Black Goat, half the cards are from the Black Goat deck and half from AH.
Plus with just one and a herald (mind you we haven't tried all combinations of one and a herald) our win percentage is significantly higher than with all the expansions in. There was one game where the terror level rose to 9, and only once did the outskirts pop. There have also been many games where we have been close to winning (usual by sealing, though at least once by closing) when a gate burst would come along and foil our plans. We've even lost a four player game in four turns, so adding in all the encounters from all the expansions does significantly (enough) lessen the chance of getting "a gate and a monster appear" encounters. Also, playing with 4 investigators is, in no way, "the hardest mode", it's optimal (unless of course you are playing with both DH and KH, then 5 is). Just as a point of reference, we play anywhere from 3 to 7 investigators.
As I said before, we've had the terror level rise to 9 with it only increasing one time due to monsters in the outskirts. Four investigators would mean only 1 monster per gate, which makes it much easier to keep monsters in check (3 investigators still means only 1 monster per gate, but playing with DH and KH means 1 investigator will at some point need to head up to Kinsport, which leaves only 2 investigators to contain monsters in Dunwich and Arkham and close/seal gates), while 6-7 investigators with all the expansions, means two monsters per gate, someone eventually needing to head up Kingsport, and needing to do a lot more monster killing (with no guarantee that any investigator will be adequately equipped to kill the tougher monsters). In one game (which was actually one of the rare times we did end up winning), my investigator - who was one of the better monster fighters - had accumulated 6 injury cards, and two other investigators had been devoured due to duplicate injury or madness cards. Generally whenever we close/seal, we're lucky if we can remove 3 monsters (including Dunwich and Outskirts), most of the time it's 1, or 0 (stupid cross symbol - though occasionally the only gates investigators can get into are ones with duplicate symbols).
Based on the above, I'd spend less time dealing with monsters and with Kingsport. Kingsport is so boring, we've mostly quit playing with it. However I have developed and played with a herald/scenario that makes it signficant:
1: Treat each pair of rift tracks as a rift track so there are no bumps. This brings out the rifts twice as fast.
2: Each time a rift track moves it brings forth a doom token. The brings out the doom tokens twice as fast.
3: Treat the monsters in Kingsport as part of the monster limit. This means that we can't just ignore them like we've done in the past.
4: Use the alternative Rift tokens found on Boardgame geek. This puts rift markers into all the expansions and into the Kingsport head.
In short, we played many, many games with all the expansions and we found we were pretty much playing the same game over and over again. I don't think our winning percentage changed much, but when went to an expansion and herald my other regulars perceivec the game as more difficult as do I? Maybe its only more difficult in that the same strategies don't work game after game. OTOH, I suppose we might develop a strategy for each expansion/herald, but with 5 expansions times 5 heralds that will be 25 strategies and that might be fun.
Also, all games include all monsters and investigator cards. I think the monsters have gotten tougher as time goes on and there's a lot more barely usable investigator cards. (There's more really great ones too.)