Is there something wrong with joins that don't use the JOIN keyword in SQL or MySQL?

SELECT a.someRow, b.someRow
FROM tableA AS a, tableB AS b
WHERE a.ID=b.ID AND b.ID= $someVar

SELECT * FROM people p, companies c
    WHERE p.companyID = c.id AND p.firstName = 'Daniel'

SELECT * FROM people p JOIN companies c ON p.companyID = c.id
    WHERE p.firstName = 'Daniel'

SET SHOWPLAN_ALL ON
GO

DECLARE @TABLE_A TABLE
(
    ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
    Data VARCHAR(10) NOT NULL
)
INSERT INTO @TABLE_A
SELECT 'ABC' UNION
SELECT 'DEF' UNION
SELECT 'GHI' UNION
SELECT 'JKL'

DECLARE @TABLE_B TABLE
(
    ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
    Data VARCHAR(10) NOT NULL
)
INSERT INTO @TABLE_B
SELECT 'ABC' UNION
SELECT 'DEF' UNION
SELECT 'GHI' UNION
SELECT 'JKL'

SELECT A.Data, B.Data
FROM
    @TABLE_A AS A, @TABLE_B AS B
WHERE
    A.ID = B.ID

SELECT A.Data, B.Data
FROM
    @TABLE_A AS A
    INNER JOIN @TABLE_B AS B ON A.ID = B.ID

SELECT A.Data, B.Data  FROM   @TABLE_A AS A, @TABLE_B AS B  WHERE   A.ID = B.ID
  |--Nested Loops(Inner Join, OUTER REFERENCES:([A].[ID]))
       |--Clustered Index Scan(OBJECT:(@TABLE_A AS [A]))
       |--Clustered Index Seek(OBJECT:(@TABLE_B AS [B]), SEEK:([B].[ID]=@TABLE_A.[ID] as [A].[ID]) ORDERED FORWARD)
 SELECT A.Data, B.Data  FROM   @TABLE_A AS A   INNER JOIN @TABLE_B AS B ON A.ID = B.ID
  |--Nested Loops(Inner Join, OUTER REFERENCES:([A].[ID]))
       |--Clustered Index Scan(OBJECT:(@TABLE_A AS [A]))
       |--Clustered Index Seek(OBJECT:(@TABLE_B AS [B]), SEEK:([B].[ID]=@TABLE_A.[ID] as [A].[ID]) ORDERED FORWARD)

SELECT a.someRow, b.someRow
FROM tableA AS a
INNER JOIN tableB AS b
  ON a.ID = b.ID
WHERE b.ID = ?

SELECT *
FROM table1 AS a, table2 AS b
 LEFT OUTER JOIN table3 AS c ON a.column1 = c.column1
WHERE a.column2 = b.column2;

SELECT *
FROM table1 AS a
 INNER JOIN table2 AS b ON a.column2 = b.column2
 LEFT OUTER JOIN table3 AS c ON a.column1 = c.column1;