Here's what I mean: you have Gortash sending Karlach to Avernus, where Zariel gets to try out her brand-new prototype engine in Karlach's chest, but the thing it's a prototype for isn't going to ever be in Avernus, it's going to be in Baldur's Gate. So who made what for whom, and what did each of them get out of the deal? Did Gortash build the prototype and give it to Zariel in the deal? If so, what's he getting in exchange for both a powerful new weapon and his bodyguard? Did Zariel build the prototype? If so, why is she trading away a powerful new weapon for a single Tiefling (and not even her eternal soul, just a warm body for the Blood War)? And why did she never improve upon the design herself?
So, obviously this is pure speculation but I think I have a reason. Spoilers for Descent into Avernus 5e module.
The thing about infernal engines is that normally, they need a fuel source to even function. Souls. Rather than just to juice them up, they're needed to power an engine on an Infernal Warmachine (a mad max type style vehicle where safety doesn't exist and everything is a weapon). Now that's all well and good for Hell where Soul Coins are a currency and while rare, you can find some consistently if you make deals/kill for them. But this also means it's VERY costly to have large amounts of them for an army, so they're mostly used by warbands, the damned and the unlucky to get around Avernus while avoiding anything they're not prepared for. Having an engine, even just a prototype, that doesn't require a soul every 1-3 days you need to use it, would be a massive advancement for the armies of hell.
Now, we do only really see Gortash's half of this and making the Steel Watchers. It's possible that other advancements are being done like implementing them into War Machines. And it's also possible that due to Hell's bureaucracy that it's being slowed down.
It IS still some kind of infernal engine, so I don't think it's too much of an impossibility for a mechanic to hear problems going with it, even if they can't fix everything.
Again, all speculation. Afterall, no one is really wondering why Karlach isn't just consuming soul coins to stay alive so maybe that's just not how they work anymore. I'm currently running the module for some friends and thought this info would be neat, and maybe explain just a little bit for some of your questions.
