What I meant is; not all factions can be compared and every faction has something that seem to be OP where it really isn't that big of a deal.
As stated earlier; if the Flak HQ is used aggresively it can easily be destroyed by a AT-gun, mortar, artillery piece or off-map ability (same way as Axis have to deal with the BOFORs).
No offense, but judging by your replies thus far and how you perceive the flow of the game I can tell you're not exactly a high rank player.
Stock mortars on their own absolutely can not destroy a Schwerer. They don't have the range or damage to keep up. The USF mortar is pitifully underequipped for this task, and the UKF mortar pit gets shut down LEIGs if you put it anywhere other than right outside your base. The only mortar that is up to the task is the 120mm, which is doctrinal.
AT guns suffer the similar problem where they barely outrange the Schwerer itself, and with the recent changes that makes the crews take more damage, you can easily lose your 320MP investment and then have it stolen away by the OKW, who will have no trouble doing so since all they have to do is walk a few meters into the protection of the flak.
Artillery pieces are doctrinal, way too inaccurate and slow to recharge to reliably damage a Schwerer, and are usually locked behind crap doctrines anyway.
Off-maps dealing with this is just...lol.
The biggest thing you don't seem to get, however, and what makes it obvious that you don't really play at a higher level, is that you seem to think everyone plops their Schwerer dead center in the map on a mid VP or some key chokepoint. Only phone-book ranked players do that. The problem is when this unit is placed defensively on a cutoff deep enough into enemy territory that you cannot attack it without a huge amount of risk, but then it still is a free defense for a cutoff, fuel, or VP, and often multiple depending on the map.
Most 1v1 maps are designed in such a way that one key point is close enough to each base that keeping it defended is more logistically feasible. This is partially to compensate the lack of early-game mobility and unit utility for axis factions - but as an allied player, a huge part of your life's blood is being able to use that mobility and versatility to harass your opponent's point. Even if it's well defended, you can still maneuver around MGs, AT guns, etc, and its this kind of tactile positioning that makes this game a joy to play and watch.
Now comes the Schwerer, which essentially locks down that already more defendable point near the axis base and makes it nigh impossible to come close to. Suddenly, flanking becomes useless since this unit not only suppresses but also kills infantry incredibly fast at max range, so a huge part of the game becomes null on a third of the map (usually). If the unit is on a VP, it allows the game to be dragged on far longer than it should under normal circumstances and gives OKW time to access arguably the best late game arsenal in the game.
Every other faction, at every point in the game, has to devote some effort and micro to both attacking the enemy and securing their more defensible points in the map. With the OKW, having this base structure lock down your defensible points effectively reduces a huge portion of your micro workload that no other faction has access to. It was excusable when OKW had a resource penalty and gaps in their core roster that made such things required for them to stay competitive, but they have since received massive overhauls but still retain this snowflake unit for whatever reason.
Once again, I'm not stressing that this unit is overpowered on paper (although its ability to model drop is over the top), but the skill gap it creates is pretty massive and glaringly obvious.
The best solution I can come up with is to reduce the lethality of the gun so it doesn't drop models at max range, and/or make the gun a timed ability - (it doesn't even have to have an attached muni cost) that way the OKW player will have to judge whether or not an assault on their point is coming and choose wisely when to have the gun open fire, and at least requires SOME micro and effort to use.