The phrase "is sometimes tricky" is grammatically correct and can be used in written English.
It can be used to describe something that can be difficult or challenging at times. Example: Learning a new language is sometimes tricky, especially when it comes to understanding the grammar rules.
I didn't know what to expect when I came here to uni – obviously I've got nothing to compare it with – but overall it's been really interesting seeing so many new things and experiencing a different culture, although coping with studying here is sometimes tricky due to communication problems.
Shipping is sometimes tricky when it comes to ladybugs as shippers want to minimize the length of the ladybugs' journey.
Ideally, one wants to place the commentary in between calls or call bouts, but this is sometimes tricky.
Finding the right color of powder to match your skin color is sometimes tricky.
Chances are, you'll have to get the hydro-cleaning as it is sometimes tricky for hygienists to manipulate around your machinery.
It's sometimes tricky to hear, and it's all head and very little heart.
Parents waiting to pick up their children from the Padre Coloma infant and primary school in eastern Madrid on Wednesday afternoon said finding the right balance between homework and family life was sometimes tricky.
We may not have had a Christmas tree and our parents may not have given us Christmas presents (another way they differentiate Christmas from Eid: my family only exchanges presents after Ramadan, whereas on Eid-al-Adha we give to charity rather than to each other), which was sometimes tricky to explain at school, but that didn't mean we couldn't still take part in it.
Reporting on the media is sometimes a tricky role, with competitors quick to complain if they sniff bias or lack of balance.
Calculating owner's earnings in a capital-intensive business is sometimes a tricky matter.
But putting scientific findings in the hands of the anarchic Sesame Street Muppets is sometimes a tricky proposition.
