I can give a bit of my opinion since I have tried all of the games Nutaku has to offer, barring Sweet State and BBE.

Aegis: No chance in hell you're getting all of the units without paying, but honestly all of the units are pretty good art-wise. You'll be able to do a pretty fair amount of the missions with mearly the 'rare' (silver) units in the game as long as you have a bit of strategy. The God level missions which are the end game content do provide an adequate challenge towards the end game. By the point you hit that you'll need to have a fair bit of event units or premium units IMO, but it takes a while to reach the point that they are event feasible. Gameplay is fun and the game has pretty low time commitment.

Osawari: Pay to Pray, in my opinion. You get a handful of good units every event if you consistently farm and are lucky, but it really all comes down to RNG in this game. From the drops to the gatcha, everything is luck unless you pay. I don't really think it is possible to get the top ranking event unit without at least a 50% booster, and they seem to have a pretty low drop rate in the ticket gatcha. I tend to get a a booster every 3 events, but that is just me. RNG is the name of the game. The roster is pretty large and the scenes are great, but a VERY large amount of the units are behind a pretty steep paywall. 100$ for 25 pulls is not very kind to those with poor luck. Be aware that duplicates are very common. I still really enjoy the game though, the story during events can be quite entertaining at times.

PeroPero: Card collection game. At the start, the events generally won't be very kind to you. It takes a while of playing and building up your level/card base to really compete leasurely for the event cards. Generally, to get every card in an event costs me around 5-10$ depending on gatcha luck. Costs can jump up if you try to get all the limited gatcha cards as well though. Progression can be very quick at the start but it starts to slow down around a month or two in. It is the game I am most committed to though, event-wise.

Harem Heroes: Great game to just kill time in. Has almost no time commitment, you get what you put into it. It is pretty new so the content is a bit lacking right now, but with new content coming out next week it is something worth trying. The theme is anime parody, so if you don't like your sexy stuff to be funny it might not be for you. Id at least give it till you finish the first world though (the pirate ship) before dropping it if you do try it though. I have to say I enjoyed it the most out of all of the games I've played recently. Not competitive at all though, its really just to have fun with in an extra tab, just jumping to it from time to time in order to add girls to your harem.

Hitsuji Chronicle: Fun game with a pretty solid list of characters. This one is a bit the same as Osawari in that it really relies heavily on RNG for which girls you get. You get a pretty fair amount of gatcha tickets from the campaign and events so you'll have a sizable cast of girls soon enough, but don't be surprised to get a fair amount of copies as well. Be aware that copies automatically fuse into themselves so you can't have multiples of the same girl on a team. The gameplay is pretty slow in the campaign, but you use up your stamina in the blink of an eye. The game has an auto feature but if you switch to a different tab the auto goes abysmally slow. In regards to stamina regen, it is pretty fair. You can power level to 30 rather quickly, and at that point the timer is ~4 hours so you don't really have to baby the game. The events are ranked, but unless you're paying to snag the top booster and stamina potions I don't think you'll have much luck getting a max evolved event unit. To get the mats for the EU's top evolution you need to place top100 which is pretty difficult. The main redeeming feature for the game though are the story quests. Each girl has their own story, 3 per evolution. In my opinion all of the stories are pretty solid, and you might find yourself getting attached to a few of the characters once you get to know them. You're much better off thinking of each story as happening in an alternate timeline though, as they don't flow together well if you think of them all happening in the same timeline.

Flower Knight Girl: Good game, one of the most popular on Nutaku at the moment. The maps tend to be pretty easy once you have 2-3 weeks under your belt. You might miss out on a perfected event unit the first time around, but by the time you get a couple max leveled 3*s and a strong friend you'll be rocking them like the rest of us. The game has low time commitment and pretty good character designs. The game doesn't really have anything particularly bad about it, barring the max difficulty IMO. Good game to pick up and auto in an alternate tab. The story can be decent sometimes, its not my cup of tea though for this game.

Idol War Z/Dragon Providence: Same games pretty much, one with idols and one with monster girls. I have played a decent bit of both but dropped both due to the shear amount of time you need to commit in order to do well ranking-wise. Good game if you don't mind being glued to it, but otherwise it might be better to stay away. Just not my cup of tea, so I'm a bit biased here.

Maybe that helps a little? Or maybe not, but good luck choosing what you want to play!