Callouses? I doubt it. Except for possibly making it painful to hit the keys. You don't need a huge amount of sensitivity in terms of feeling the keys. Longer finger nails? Depends how long and how you finger comes in contact with the key. I still doubt it.Disadvantages? Splitting your time between two instruments. Having to develop the technique and sharing the same body parts -- The fingers might get worked out, the same parts. But that can be cross-training in a way, not necessarily bad. Maybe trying to learn tabulature along with a grand staff.Advantages? Being able to take a break from one instrument by playing the other. Sitting with music for longer than you might have with just one instrument. They're both percussive. You'll get better ears for intonation with the guitar if you're tuning it a lot.
Starting guitar after having played 10+ years of piano is DIFFICULT!...Alternate picking while moving my left hand around different notes for each pick is still way beyond me; and it's kinda frustrating because when you start piano you're exposed to [simpler] music... I've seen all the craziest stuff people can do with the guitar; imagine just learning your C major scale and seeing Chopin Op 10 #1... really bogs you down lol.In other words I suck at guitar lol.
At the very start, you should practice A LOT of technical exercises focusing on each hand. I don't know if there's a way around this like in piano where you can learn pieces to develop technique.
Building technique via learning pieces works just as well on the guitar as piano. I got my guitar technique from learning pieces, and have only ever done one exercise which wasn't really that helpful...
I think electric/ pickstyle guitar is the hardest there is, if you count shredding techniques and two-hand tapping. Especially if you're so used to piano, I recommend fingerstyle.
The irony is there are a lot of teachers who are against exercises for piano, but I have never heard of one for guitar. Maybe it's because most guitar students hack a lot of songs without doing their technical stuff, while piano students are more obedient with regards to exercises, so much that they rarely play music??
Or is it because for piano, HS practice and slow tempo practice is common, while for guitar, very difficult? (Imagines a guitar piece for RH or LH alone... And holding barre chords for 8 counts in 60 bpm... )Or is it because there is no Hanon and Czerny in guitar literature? (Imagines books called The Virtuoso Guitarist and School of Shredding... )It's too late for me... I have already spent 3 years of my guitar life doing exercises.
I've played both electric and classical guitar, and without question, classical right hand technique is far more difficult than using a pick, as you have to be able to play 2-3 different lines simultaneously, sometimes in different rhythms (at least pianists can separate these elements between their hands!). Sure, a good electric guitarist can play more notes per second than even some pianists/violinists, but they are playing only one line at a time. I used to be impressed by "shred" guitarists until I learned some myself...it's not all that hard to do.