For myself, the definition of a "F2P-ness" of a gacha game is measured and defined by how it rewards effort x time with the gacha itself. So shit like
this or like
this is out of the equation. If for example you were able to farm a specific unit once a month (30 days) as a hardcore player or 1.5 months (45 days) as a semi-casual through in-game currency or materials, I would be perfectly fine with it. At the end of the day, I am playing a
game and not trying to become wealthy by investing into a game. I would also be willing to compromise with some sort of pillow system where loyalty (logging in for x amount of days) guaranteed you for every 30 days a unit of your choice or for every 1-3 months a paid package of similar outlook.
Irrespective, the best option is to always have a pity system.
With some quick research, both gachas have got brutal gacha rates so where is exactly the safety net of gacha-pulling satisfaction?
Well Destiny Child has a 3% 5* rate which I wouldn't exactly call "brutal". It also showers you with premium currency (from the dailies alone you can get 900 crystals and a 10x pull costs 2700). There are events going on nonstop that either reward 5* tickets, copies of the event Child, or crystals.
If you play long enough it's more than feasible to have the entire common pool roster, at which point you either pull for Boost childs for events like World Boss or for dupes to unlock higher skill grades. The maximum uncap a Child can have is +6. I'm fully F2P and have no issue being in the top 5% for World Boss damage, which uses 20 Childs at once.
PvP is highly centralized - 3 specific Childs are more or less mandatory the higher up you go so you only have leeway in two slots for variety.
Nevertheless there's a reason it's commonly recommended alongside AL and GFL as one of the most F2P friendly games. The packages and Devil Pass are comically overpriced so I don't bother with them and never have, but nothing is giving the feeling that you really need to spend at all, unless you want to be at the top of arena (Devil Rumble).
The big draw here for me honestly is the fun dialogue and zany events - I think the story is underrated. Planning how to squeeze more damage out of my roster is enjoyable too.
Brown Dust on the other hand is
hard PvP focused. It has multiple arenas - Novice (no 5*s allowed but highly centralized), regular Arena (anything goes), Underground Arena (live PvP, essentially you take turns putting Mercenaries on the board), and Guild War.
If you are not interested in PvP there is very little here for you and the rates are not great. It does have Mileage, but it takes quite a bit of it to turn it into a 5* selector ticket. Merging goes up to +9, but you can either use dupes or Books, and you can get about half a dozen of them per month.
The bigger problem are the Legends, particularly the Octos and Six Devils. They have their own legendary Books and you get fewer of those. A few months ago they have introduced another system called Companions, which essentially need to be pulled from the gacha, and allow some Mercenaries (including soon the Six Devils) to be merged up to +15, which hugely shifts the metagame again.
There is an event coming up where you get a +9 Six Devil, and IIRC four more +9 5*s for free, but they'd still need investment. I feel like if a game needs to go to such great lengths to give out something for free, it's having problems.
This is honestly more of a competitive whale's game. F2Ps
can compete, and the strategy goes rather deep, but you have to stay on the ball extremely hard, know exactly what you're doing, and allow for no leeway at all.
That's why some see it as F2P friendly (game knowledge is a huge factor and the biggest hurdles are time gates, but you can compete at the somewhat highest levels, as well as clear any kind of PvE), while most deride it as P2W trash for the hard PvP focus and catering to competitive whales.