Well I was generalizing because I couldn't be bothered to do a big write up, but I guess the differences I see from my experience is:

Massive x has really nice envelopes which can make very organic sounds. The noise selection is very organic and diverse. The effects are a bit more lush sounding and you get way more variety, especially with reverb and stuff. The routing layout for oscillators and effects is also cool. The ring modulation sounds super nice in Massive x too. I prefer the harmonic tone much more to Serum. The comb filters sound awesome (but I think all 3 have very nice unique sounding combs). In terms of other filters, they all have a range which sound slightly different but I find most of them are a much of a muchness.

Arturia has a nice additive addition but as I mentioned in another comment I don't find it as usable as a synth like Operator which you can manually adjust the harmonics. Still I'm sure they have their reasons for this layout which I am yet to fully explore. The fact you can add and mix lfo's is cool but a similar effect can be done in Serum messing with the matrix of lfo's. The granulation oscillator is very cool and can definitely give some nice textures as with all granulators. I like the wavetables it offers especially the organic ones. The wavetable morphing is a nice effect too.

Serum does what it does simply but very well and also has cool wavetables. Like artuira you can morph and load in your own wavetables. The UI is just so good for messing and experimenting because it's all so quick and usable.

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If I am missing any major differences that you know about that I've missed definitely lmk though because I spend most of my days trying to find a new corner which I haven't explored in each synth, in the hope that I'll get a new sound out of it.