Welcome to the JavaRanch, Amlan!
Your most immediate problem isn't
JSF. You're violating the database constraint that the "comment" column cannot have a null value when you persist it out.
As far as ui:repeat goes, yes, commandButtons can be used in ui:repeats, but I strongly recommend that when you construct 2-dimensional (table-like) objects on a JSF view that you use a dataTable, not ui:repeat. Usually when you have a table full of rows with action buttons (and/or commandLinks), the entire table is encased within a single form, however, not form-per-row.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.