Whilst learning a new instrument can be extremely rewarding, there are also some disadvantages.
‘Jack of all trades, master of none.’ I’ve met many musicians that play every instrument under the sun, but I have never met one who excels at one or all of them. When you need to practise 3+ instruments you can end up spreading yourself too thin. It takes a lot of time to reach a high standard on an instrument and we’re all busy people. Say if we only get roughly 45 minutes of practise a day, rather than focusing on one instrument at that time, we’d have to split that between multiple instruments. 45 mins a day, can turn into 15 mins a day, which will stop you reaching your maximum potential as a musician.
The other issue you might run into is that if you could end up confusing yourself, say if you learn to play something like the trumpet or saxophone, which is played in a different key to the guitar. You could get confused whilst you develop the theoretical knowledge needed to understand the relationship between the two instruments.
I found this blog hard to write, because personally I believe learning another instrument is a good idea, as long as you can either balance your practise time effectively or you can prioritise what instrument you want to play and you don’t learn too many. I just wanted to warn you that it can have a detrimental effect on your music!