I have read someone suggested HBS is known for making a good base game and weak expansions and imho Battletech would rather be proof of that they stuck all in for the base product and ran out of creative fuel afterwards with each expansion weaker than the former.
I don't know, stepping back after the fact and looking it over? I don't know if it was that... or just not having the resources to go as hard at the expansions as the main game. Remember, the main game was under development for considerably longer than each of the three expansions... and there was a "free expansion" usually developed alongside each of the others.
I'm going to write a lot here, as there's simply a lot of things to talk about to illustrate this point. Keep in mind as you read, the main game was released on April 24, 2018 with only the Skirmish being shown as "finished" on June 1, 2017 to Kickstarter Backers. (Which are many people still kicking around here, who got together and realized headshots are fricking deadly.)
Flashpoint (August 21, 2018)
Flashpoint had a pretty neat new addition, in the form of "Flashpoints" - something the other two expansions would also work with. This being a new mission type of linked contracts in a short storyline... it was very fun to see them unfold and mostly a nice break from "just flying around sniping Assault 'Mechs in the face". Alongside this, there was the free update for 'Career Mode', which altered how the game was played. Significantly. (I say "alongside" but if I recall correctly, CM was released before Flashpoint - but had to have been worked on within the same timeframe.)
We got some new 'Mechs, two of which were rather special: the
Hatchetman had it's killer melee swing, the
Cyclops had an Initiative-changing device, and the
Crab was an energy-only 50-ton 'Mech. Each of these needed to get tested, and the
Hatchetman had to be designed from scratch as HBS was still using data from MWO - to make it possible to get the game out in the late 2010s instead of early 2020s. But as MWO didn't do melee combat, the
Hatchetman had to be built in-house. This, along with developing the Jungle biome, probably kept a good chunk of people busy.
This was released just four months after the actual game, and this timeframe also included the bugfixes necessary at release once the game was "released into the wild".
Urban Warfare (June 4, 2019)
Urban Warfare was released around 10 months later, bringing a small selection of new 'Mechs but also bringing urban combat into the game. We only got two new 'Mech chassis in the
Raven and the
Javelin. Along with those, we got a number of variants models (seven in total) which were interesting variants - if I recall correctly, all except the HCT-3X
Hatchetman were free additions. Now, while finally getting Urban biomes was a major thing, but it also brought EW equipment into the game.
I will need to stress here - this development was one reason we were told the
Raven was not going to be in the main game. Any of us who play tabletop know this Capellan tech is basically a block of useless weight in 3025, and winds up outclassed when the Helm Memory Core gets released. There needed to be something to the equipment which made it worth picking the
Raven. I'd be willing to wager the development of the EW items were part of what really took time.
Heavy Metal (November 21, 2019)
Moving ahead, we see a five month gap before Heavy Metal dropped, and the big news here was the Unseen. I don't think I can understate this, the Unseen 'Mechs are a massive thing to have make it back into the game. Two of these (The
Warhammer and
Marauder) were promised at launch, but legal BS meant they were quietly yanked. But we'd finally be getting them, and HBS doubled-down to get us three others: the
Phoenix Hawk, the
Rifleman, and the
Archer. And they all got unique equipment-based quirks!
And there was an entirely new 'Mech designed in-house called the
Bull Shark. And there was a miniature campaign of linked Flashpoints which would explain where all the Lostech goodies were coming from. (In a loose sense...) Oh, and now that we have the
Warhammer and
Marauder, we can finally have the Black Widow and Bounty Hunter in the game using their iconic 'Mechs...
All of this in five months. I'm not surprised it looked "feature-thin".
Shortly after that we got the "
we're done, we totally mean it this time, we're not adding anything more" 1.9 Free Update. Sixteen new variants, and some tweaks to matters.
I don't have the comprehensive list, and if I did this post would be twice as long. But HBS did their work and the only notable gap was before the expansion with the "stealth mechanic" which was - even on release of it - problematic as all heck. (It STILL can fluster the AI something terrible.) These folks burned through everything they could, trying to keep the game constantly fresh. This isn't a Triple-A studio, and BattleTech wasn't going to be mainstream, they needed to be consistently keeping it active so people could hear about it for the longest period of time without it "cooling off". Once an indie game buzz gets cooled off, it takes a LOT for it to get pushed back into the light again.