Guitar is pretty easy, at least compared to piano. The problem with piano and many instruments compared to guitar and its relatives, is that you have to relearn each scale, or song, or pattern, or lick, or whatever every time you change keys. But with guitar, if you learn something and want to play it one semitone higher, then hell, just move up a fret. This is kinda nice and handy when yo are a beginner. If you can play a g-major scale on the guitar, then you can play any major scale starting on a note on the same string. With piano or, say, saxophone, you have to learn how to play 11 separate major scales to cover every key.
Having said that, if you are serious about music, piano is the best instrument. ALL serious music theory courses use piano as the sample/demonstration instrument. If you take music in college or university, it doesn't matter whether you play viola or marimba, the music theory class will use a piano. Not only that, but any incompetent clown can hammer out Smoke on the Water or the opening bars of Stairway to Heaven on the guitar, but with no more effort or talent, you can be the smooth motherfucker that can play the opening bars of Lovin Spoonful's Summer in the City. You'll be a hit at every party. Jackasses with their accoustic guitars won't know what hit them as their girlfriends all flock to your side. You will be rich and successful, that acne will clear up and everything will come up roses I tells ya.
So easy, a monkey could do it. A smooth motherfucker of a monkey!