I'm new to the Helix, but not new to multi-FX pedals. In the past I've used multi-FX pedals (Boss GT pedals to be exact) in 4CM with tube amps. Overall, I'd say that I've been very pleased with the results. Now, with the Helix and the Line6 Powercab - an FRFR speaker - I'm going full modeler mode. The UI is great - I think it's much easier and more logical than the Fractal and the Kemper gear (which I have owned and tried). My challenge is getting the amp and speaker / IR settings just right. The Helix FX are great. If, for whatever reason, I get dissatisfied with the Helix amp modeling then I'll use it in 4CM mode as a replacement for my old GT units. As far as comparing the sound quality of the Helix vs Fractal vs Kemper vs Boss vs....etc...., once you get into these higher end units I think they all have the ability to sound pretty great. Each one will have it's pros / cons and strengths / weaknesses - but I think we're talking about very slight differences. So, for me, a major deciding factor was also the convenience and user-friendliness of the unit.
Generally speaking... How can you not love a multi-FX unit? I really don't get the individual pedal thing. Individual pedals and all that they require (additional cables, board, power supply) are more expensive, less flexible (you can't have preset patches of different combinations of FX with unique settings), are likely to be far more limiting in terms of FX options (unless you have a pedal board as big as basketball court), and with every additional pedal you're introducing another potential point of failure, noise, and / or signal degradation into your sound. With today's technology and options, a rig with a bunch of individual pedals has no appeal to me.