"better" is a dangerous term. Better as for bruteforce guessing resilience? Sure. Better for security? That would be much less certain. Once you discover the users can't enter it on mobile (no Σ on default keyboard) you'd have to provide replacement mechanism for them or allow them to easily reset passwords. Which probably would be the way your security will be compromised soon. And, of course, the users will be losing such fancy passwords at incredible rates, which would generate a lot of noise on "password reset" system which would make it harder for you to detect abuse there. And that not even considering that humans are very bad at avoiding biases and choosing "random" character replacements - they probably would do the same thing in 90% of cases (yes, you are in the 10% of course, but that doesn't change the statistics), like replacing o with 0 and adding ! at the end. Which robots would defeat instantly.

So it's not about forbidding you from using your special characters. It's that using them - on the whole, not for you personally - may not make any improvement, and if you force it, might actually make the matters worse. Encouraging long passwords that one can remember would seems to be actually better. Or doing away with passwords and using something else :)