Yeah it's an interesting topic... and of course pros and cons to each depending on your specific intentions, goals and trajectory of training.
I personally think certain arts are more hmmm how shall I say... designed to train and learn deeply. In that it takes a lot more effort and time to become proficient. And I appreciate and follow this approach more in my practice. Staying within the one art, yet still learning from other viewpoints allows me to go deeper within my own art, other perspectives enriching and allowing different practices to emerge.
It also goes with how I view something else... I often hear people say "nah you've gotta be a more well-rounded and balanced martial art artist", but my view of being a balanced martial artist is different, and doesn't relate to just being good at everything in martial arts. This is fine, but I feel balance relates to a depth of holistic training within your path, not in being good at striking, and grapping, and groundwork, and weapons etc etc.. that appears to be adding more 'stuff'. Again, this is completely fine. You learn alot technically here, but it's possible to be information overload simply because there're more things you can always add. This may be school dependent of course in terms of integrating it all whilst still being able to delve into the foundations and developing depth and so on.
But sorta like that Venn diagram I see that depicts balance as being equal measures of health, finance, hobbies, social, family, spiritual etc. I don't see balance like that, but as an ever-shifting necessity that either relates to need or curiosity. Sometimes I need my priority in a certain area, so this area is emphasised or prioritised in order gain more balance across the board. It's more a positioning of your time and attention that helps all others grow too.
For example, I trained many many years in a very hard style. It took its toll, so I had to seek out something softer, but still with harder elements. In really emphasising the soft I could truly appreciate and gain more understanding as to the harder stuff, and in a more balanced and healthier way of approaching it.
Anyway haha, being more well-rounded in martial arts to me means being open to where you need to develop, evolve, integrate and consolidate within your chosen art. Not only a focus on adding stuff, but in exploring the same in more depth, and applying it to different contexts and parameters. And in this way it's not merely a "master of one", but a pathway that's deep and fulfilling on alot of levels. Rather than accumulating its focus is refining and deepening.
I have no idea if I made any sense haha but here we are!