The error is because of a difference in character length of the two "name" fields. Vertical concatenation of characters into a character array requires the same number of characters, even in a table.
Use a cell array of characters, a string (shown below), or a category instead.
a = struct('id','a','mode','read','name',"firstname",'type','double');
% string ^ ^
b = a;
b.name = "differentfirstname"; % string, not char-vector
Ta = struct2table(a);
Tb = struct2table(b);
[Ta;Tb]
