Del is better, Kat is a promotion and as such here pool is more shallow lacking some key perks such such as reduced Fortification Cost Reduction, Weapon Locker Ammo Refill and Sentry Ammo Refill Cost Decrease. With that said Kat is perfectly fine with a few unique advantages. Del’s Ultimate is fairly weak off-set by the fact that unlike some characters he gets it back fairly quick, with Kat you don’t have to sacrifice much to get her Ultimate even faster from repairing whereas Del would and his pay out is meh at best. Kat’s Ultimate has decent utility basically a free decoy for a short a while which can help her get up front to repair broken fortifications to stop a charge or what have you. Halo which Emile also has gives you some added defense for the start of each wave (unless you skip the time in-between waves though there isn’t much point to do so as the Score benefit is pretty minimal and it just eliminates time you might need for repairs and the like. Del does indeed get a somewhat similar version of with Healing Repair, but IMO Kat’s is better and there’s really no reason not to take it. The buffer of Stim can save you if you’re still repairing during the start of the waves or if you take a rear position where you aren’t getting hit you can generate the buffer during the wave combined with her decoy for in-wave repairs where Del would be putting himself at a greater risk. Kat’s starting Kit is superior to Del’s as well IMO, but that’s minor since you can always ditch them for whatever you want since you’re not getting bonuses for any weapons, but it is still a plus for Kat’s side.
They did it because they’re promotional it has little to do with pay2win and more to do with fulfilling an obligation of what was advertised and not putting work into it past that as anytime they spent working on promotional characters would take away from working on their own characters. The limited sets make them perfectly usable and fulfills advertisement that they’d be usable in Escape and Horde. Their mileage varying and tending to fall short one way or another does help keep down blatant jealousy of those who do not have them sure, but it’s more a side-effect than a specific intention. Think of it this way if you’re working on a science project with somebody else and you’re getting graded on what you do and the project as a whole you might help out your partner with their part so the product is overall better, but you’re going to prioritize your part first and foremost. TC is likewise going to prioritize their own characters and not what essentially amounts to an advertisement.