I was struggling with connecting the sync combos but it wasn't that big of an issue overall. It was more the latter case, where it just felt "off" in general. And the sad part is that I kinda feel that it's on me and not on the game. I can see that Takahashi Taura and his team have put some thought into the controls. I really think I am just too inept, lol. It also took me a while to get comfortable with controlling your Legion and your character on both sticks respectively, especially when you have those jump sections... I was even more struggling when I tried to pull off some "advanced" stuff like de-chaining one Legion, then pulling out another and also controlling your character. I never felt like I had the game "under control", so to speak.
I have never experienced this before in an action game, so I don't know what it is.
EDIT: It also didn't help that I got fairly bad ratings overall... Even when I felt like "hey that went OK this time." Apparently not.
The ratings can be brutal when its your the first time doing the mission, the only way to get the S+ is to get the sync attacks connected as much as possible, and that means never being slowed down or took damages.
I think the key is to just master doing Perfect Call, a "parry" that can counter on all attacks + stun, unlike dodges. After doing ample practice with it, I just stopped moving much in battle and wait for the attacks, unsummon my legion if needed and do the counter. The move is kinda perfect to start any kind of fight. Hiding and flying enemies can easily be countered anyway with beast howl.
Another thing is to always match the combo sequence with how you click the buttons, no wasted movements, no need to press the buttons more than 3 times if that's all you need to complete the combo. Don't move much if not needed, use your legions more to move.
Also the hit rush thing is hard and I've never mastered it, only doing it once in a while for fun. Endgame I just use Axe legion with omega++ armor (no flinch) and just complete basic combos - they hit hard at the last attack, I can kill most of the mini bosses around 2 mins max, of course with dodges and perfect call.
Other people would suggest exploiting slow attacks crit rate etc, but not getting damage and being interrupted from completing combos is the first thing to be mindful for.
Also never start a combo if you can't finish it, you'll get what I mean.