It might be because he's relying on coupon spending, as I do myself, when you play like that you must plan ahead & pick up ninja worth picking up that will help you in PVP in the short & long term, things that can make an impact in the short term so you can then focus on spending on Power more so than ninja, Ninja collecting for the sake of Ninja collecting is pointless, people should only do it if they have the means to do so & really want to.

Fusion does nothing for him without the required ninja, he'd need to pick up Angel Konan, or Deidara to fully make a half decent team, Fusion is not a carry ninja by himself like Shisui is.
Madara summit is a crazy ninja, but again is a ninja that does more than Fusion by himself but still requires a carry ninja along side him, he's a support that cleans buffs & shields for a carry Ninja to then destroy Round 2.
Hinata Summer is ight, but still, Shisui is better & allows him to bond Mystery Itachi Anbu(which will then become Hinata Summer) which will compliment Shisui way better than Hinata can.

Shisui you can utilize him very well & he's a very strong ninja by himself due to him having a barrier, he'll never been fazed out of the meta purely because he can clean buffs, chaos & has a barrier, he's utilized in a few teams because of the barrier alone allowing him to support as well but also dish out damage and CC with a mystery, even as a f2p, he doesn't require anything, you can rock him with Kurenai & Iruka if you want, Itach Anbu was free & also goes well with him, then he can follow that up with Bond Mystery Itachi Anbu.