While I'm waiting for the virtual cards that apparently didn't make it to download on my Switch 2 again, I'm going to talk about Time Stranger again, because I think I've figured out how to articulate something.
Games operate on principles of reinforcement. Primarily, positive reinforcement. You do your battles or tasks, and you receive money, experience, etc. Monster catchers have the added value of things like new acquisitions and evolutions to add to the mix.
An important thing to consider with design of these system is rate of such reinforcement. The ideal is that, early on, reinforcement rates are high, to encourage continuation of the behavior (ie, playing the game). This is something easily recognizable in gachas; you are showered with resources early on, and then it fades and expects payment to continue access. Eventually, things level off, and the rate of reinforcement is either slowed down gradually enough that players persist, or it goes too low too fast and loses people.
The exact rate is a subjective assessment, but you can probably pick up a sense for it yourself. For anyone who's played the GBA to DS eras of Pokemon games, you're probably familiar with the midgame slog. Your creatures have fully evolved, you have all the pretty good moves you need, experience is slower to accumulate, and the longest routes between major gyms exist in this phase. My wife dropped off playing LeafGreen in this phase, because it just went too low too fast. She couldn't maintain interest.
Digimon Time Stranger's own systems kind of work against it in some ways. While the limitation of Agent Rank blocking off further evolution upward is nice to keep focus on your initial forms, the problem with this is that it functions as a limit to total reinforcement for playing. You're not getting the thrill of evolving to new things. Pokemon's extremely early first stage evolution vexes me because I have come to generally like the first stages best and would like more time with them, but giving those first evolutions super early is wise; it gets players invested in the development.
But Time Stranger also limits de-evolution. Unless you share a personality, something that takes considerable time and RNG luck to achieve if you weren't lucky enough to already be there, you also can't de-evolve to things you haven't seen before. So you're gated on both ends.
Acquiring new Digimon also tapers off after literally the first dungeon. The tutorial introduces that slime-level Digimon take two encounters to their best stats, and the phase right above that takes only four. Immediately after this dungeon though, most encounters are back to the 15-20% per encounter rate; a substantial dropoff basically right away. This means that only one out of every ten encounters with that thing specifically yields a reward in the form of your new find.
This isn't necessarily distinct. Considering Pokemon again, you can go a lot longer than ten encounters looking for your low appearance rate favorite in the grass. But in Pokemon, battling through every encounter yields a lot of EXP and a lot of levels. In Digimon, EXP scaling is sharp, and the rewards are significantly limited.
But wait! Digimon offers other avenues of reward from battles. One is just straight money. Similar to Nexomon, you get cash for random encounters. That's useful! Except it takes a long while for encounters to give more than a couple hundred Digidollars, and costs of materials is sharp. In the early game alone, there are items necessary for specific items that unlock new evolutions, that costs like 1200 a pop. The most lucrative encounter in this phase of the game is the gold poop guys, who gives like 120. Again, one in ten gets a reward. You need four of one kind of item, and three of another, to get everything. Comparatively, Nexomon gives plenty of money for your Traps and healing.
Well okay, but what about personality conversations? You can get special conversations with your Digimon that can augment their personalities, giving access to better stat spreads for their role, or potentially unlocking desired abilities. Except all of this is wildly random. Every four fights, you get a chance to do this, and for any new Digimon, you need two answers in the same direction to change personality at all. The personality skill situation is even more bleak, with Digimon seeming to learn a skill once every ten conversations (based on my experience of consistently soft resetting for an attempt), and then have a stated 10% chance to learn the rare skills that are often the desired ones (Great Embrace, Magic Theft).
But these also improve Bond! Literally half as substantially as feeding them in the farm, and the feeding option is a minuscule fraction of the time. But that costs money (50k per Digimon to be precise), and it does nothing else. It's the best, most efficient handling, but it doesn't connect to the personality adjustments or skills. You can do that with training equipment, but that's either a sizable real-world time requirement, or costly (10k per training).
Even boss fights are not immune to this. Barring the major fights after timeskip, bosses give astoundingly little EXP and gold, and even after timeskip, some of the roughest fights still give only around 2k EXP at a time when 20k wouldn't get a level. You're often better off leveling by having them eat other Digimon, but see the issue of how slow it is to acquire Convert status.
But what about moves? Maybe as you level, you can get new moves! This is partially true, Digimon do have access to some moves as they level. However. If you have enough of them going in different type directions, you have enough options in relatively short order, and then moves are just a mix and match affair; nothing is uniquely exciting in leveling up your mons. Improving stat allocations, learning personality skills, making money, learning move sets, hell even evolution access becomes more about Agent Points pretty early compared to leveling. But fighting a bunch of battles is necessary to get the creatures in the first place.
Time Stranger's system just feels very low-reinforcement to me.