Generate Key by referencing same table?

Question

Is it OK to generate a Key by referencing the max + 1 of the same table using a trigger? Can anything go wrong? (I am fully aware that a sequence can be used instead but my co-worker is trying this other approach.)
Sample Code:
CREATE OR REPLACE TRIGGER trgTableA BEFORE INSERT ON TableA REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
DECLARE
iCounter TableA.SEQ_NUM%TYPE;
BEGIN
IF INSERTING THEN
Select max(seq_num)+1 INTO iCounter FROM TableA;
if (iCounter is null) then
          :new.SEQ_NUM :=1;
     else
          :new.SEQ_NUM := iCounter;
          end if;
END IF;
END

Answer 1

e.g.
SESSION 1:
dw2@DWDEVA> DROP TABLE test;
Table dropped.
Elapsed: 00:00:00.00
dw2@DWDEVA>
dw2@DWDEVA> CREATE TABLE test
  2    (col1 number CONSTRAINT test_pk PRIMARY KEY);
Table created.
Elapsed: 00:00:00.00
dw2@DWDEVA>
dw2@DWDEVA> CREATE OR REPLACE TRIGGER trgTest
  2  BEFORE INSERT ON test REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
  3  DECLARE
  4    iCounter  test.col1%TYPE;
  5  BEGIN
  6    IF INSERTING THEN
  7      SELECT max(col1)+1
  8      INTO iCounter
  9      FROM test;
10      IF iCounter is NULL
11        THEN :new.col1 := 1;
12      ELSE
13        :new.col1 := iCounter;
14      END IF;
15    END IF;
16  END;
17  /
Trigger created.
Elapsed: 00:00:00.00
dw2@DWDEVA>
dw2@DWDEVA> INSERT INTO test VALUES(1);
1 row created.
Elapsed: 00:00:00.00
dw2@DWDEVA> SELECT * FROM test;
      COL1
         1
Elapsed: 00:00:00.00
dw2@DWDEVA> INSERT INTO test VALUES(1);
1 row created.
Elapsed: 00:00:00.04
dw2@DWDEVA> select * from test;
      COL1
         1
         2
Elapsed: 00:00:00.00
dw2@DWDEVA> --no commit
SESSION 2:
insert into test values(1);
-- waits for session 1 to end transaction
SESSION 1:
COMMIT;
SESSION 2:
insert into test values(1)
ERROR at line 1:
ORA-00001: unique constraint (DW2.TEST_PK) violatedThis won't happen with sequences