The Clena uses what I call the traditional EnerGel barrel. It’s the one you see in the wild and go “yeah, that’s an EnerGel.” The paint job is different for the Clena, but for all intents and purposes its identical in size, shape, clip, grip, and knock. The barrel is great for wider writers like the 0.7 mm size, but I feel that it is too wide for 0.3 mm. Since you aren’t generally writing big, flowing lines with pens this fine, you want the control of a narrower barrel. Not Slicci thin, but Euro thin.
Secondly, the only ink choices are black and red. Even the Euro had blue! The 12 shades of standard EnerGel ink would be perfect for the Clena. I’d even forego complaints of the barrel diameter if I could get orange, blue black, purple, turquoise, and more in 0.3 mm.