You can't just pick a DLC with 50% negative reviews and low number of review and say lots of happy customers are just not reviewing it yet. If 500 extra reviews pop up this week, it'll just as likely to stay at 50%. We are all decade-long fans of the IP but we can't pretend a bad score will change if we just have larger sample size. Short of a malicious trolling campaign, steam reviews are remarkably consistent beyond a certain sample size.
Here's an example I've been plugging for weeks because both are so similar in genre and time frame. Battle Brothers's base game got an excellent 87% positives over 5k reviews. Their DLC, which came out the same time as FP, also got an 89% positives with 250 reviews. THAT, is how DLC suppose be received compared to the base game. So is BB's DLC player base happier with their purchase compared to FP's or is BB's happier portion of the player base just more vocal?
I'm not saying FP is bad, I'm saying whatever the combination of content, quality and price HBS decided was appropriate definitely fell short of general expectation compared to the base game. If Urban Combat is just more of the same (3 mechs, a map, a new mission type, 20hrs of FPs), then things are just going to get worse.