PSA : Do not take the Cross-Platform compatibility mortar upgrade
TL;DR : Allies can't know whether you have the upgrade or not, so they will trigger target prioritization on the wrong targets. There are many times where you want yourself to mark a dangerous target for your allies... But not for your mortar sentries which will be inefficient against them and sometimes even dangerous.
For you all mortar lovers, I advise you to NOT take the mortar upgrade that allows mortar sentries to prioritize targets that you have marked. This is a trap made by the automatons, leading us to believe that software communitarianism will help us win the war. Here is why this is a (very?) bad upgrade to take.
Your mortars will target anything marked, even if you don't want it to
First and foremost, know it is EXTREMELY likely that all marked targets by ALL your team will be a priority target for your mortar sentries. The issue is, most people you will play with won't know you will have this upgrade, leading to a very high probability of -dismemberment/concussions/electric shocks- waste of ammunition. Moreover, you may not want your mortar to shoot at everything you mark, from incoming patrols you don't want to alert while you're already engaged, to targets you marked just to tell you will take them down.
Most marked targets are bad targets for your mortars
Indeed and here's the second point, most people mark heavy targets rather than small groups. This unfortunately includes tanks, bile titans and chargers, all of which are fundamentally immune to EMS mortars (read my compendium of bugs, section stunning weapons to learn why chargers are virtually immune to EMS mortars). And outside tanks, both bile titans and chargers are poor targets for explosive mortars, the first one being above your mortar's area of effect if it misses -and it will miss-, the other closing in on you too fast on you for the mortar to hit it (and leaving a deadly trail behind it, too).
Also, as sentries can target factories (read my bug compendium, sentry section :) ), you are likely to have your mortar shells wasted on something that isn't a short-term threat if any of you mark a factory. Even if they could damage or/and disable them, helping you survive by taking down enemies attacking you1 should be their top priority, as dead helldivers cannot conquer for democracy.
Then, even when the target is good for your mortar (heavy devastators, cannon hulks, groups of small/medium enemies...), all fellow helldivers will (normally) prioritize their targeting system on them, wasting potential mortar shots because it is likely to be taken down in seconds before the shells land. Don't overkill your enemies by making everyone shoot at the same target. Instead, allow your mortar to be the 5th gun in your group, let it hit enemies you cannot easily reach (ie. mark) instead. This is the true, democracy-spreading cross-platform compatibility you will ever need.
You will not be able to cancel the upgrade if you don't like it
Finally, understand that if you do take it and find that you don't like it, it will be too late. In the current state of this game you cannot disable upgrades you don't want.
It's the same issue with the artillery area focus upgrade (named atmospheric monitoring) : If you take it you specialize your stratagems rather than actually strictly improving them. And in this case, it's more often of a burden you will carry for the rest of your games than something most people would want to have.
Conclusion
So don't fall for the trap automatons have set for us. This is a virus that will be embedded forever in your mortars' AI that should be forbidden by both the ministry of science and truth until they have removed the malware. In other words, when devs will allow toggling it on/off first with, and then when they rework their whole high-priority sentry targeting system to be more clever. From my game-designer point of view, this is currently a false good upgrade with many issues down the line. Issues that won't be fixed after one month at the very least, because such improvements should/will be done after fixing crashes and softlocks for the programmers.
So remember what General Brasch said during his speech on Vernen Wells after he freed the children : You don't need technology like AT-mines or AI-assisted targeting systems to win the war, you only need courage and bravery. So be brave, helldivers! Be a true Hero!
1 Minus enemies in melee range. Being close to an enemy is not being allied with said enemy, overzealous mortar \^.)