What are the pros and cons of the Sherman medium tank?
The pro side was that they were cheap, easily produced and the US
was able to put many of them into the field quickly.
The con's include the lack of firepower and armament, the lack of an
adequate defense and because they were fueled by gasoline they lit
up like torches when hit. They were called Zippos because they were
guaranteed to light up every time.
In addition, the Sherman was a Medium Infantry tank - its primary mission was to support infantry, not go head-to-head with the much heavier German tanks (the Panther and Tiger series). As such, it was intended to attack fortified pillboxes and strongpoints, provide fire support, and counter light armor. It thus had a medium-velocity mid-size gun, which proved excellent for use with high-explosive rounds (which, against masonry and concrete, are highly effective). The lack of sloped armor on the hull sides made manufacture much simpler, as did the rotary gasoline engine (which was simple to make, and even simpler to service in the field). Finally, its light weight meant it could easily use bridges which could not support many other tanks, as well as making it easy to adapt for a variety of unusual services that heavier tanks were unsuitable - floating tanks and anti-mine tanks, are some of many adaptations made to the Sherman.
Thus, the Sherman was optimized for very high rate of production, ease of service in the field, and for use in a scouting and infantry support role.
Unfortunately, the Sherman frequently found itself pressed into service in roles it was entirely unsuitable for, particularly when put up against much, much heavier German tanks and high-powered anti-tank weapons. The armor arrangement of a Sherman was suitable protection against up to 60mm medium-velocity or 40mm high-velocity rounds (as those found on the Panzer III & IV series), whereas the Panther and Tiger tanks used 75 or 88mm high-velocity weapons (as did the feared 88m anti-aircraft gun). The gasoline engine (a benefit in the cold weather vs a diesel one) quickly became a huge liability there, as the insufficent armor protection meant that a Sherman would catch fire quickly. In addition, the Sherman lacked a proper ammunition storage system, as most rounds simply sat on the floor. Naturally, this is a horrible thing when the armor is pierced - almost as many Shermans blew up from a hit as caught fire.
Similarly, the original engine in the Sherman was of sufficient power to move the tank at good speeds; however, as serious field modifications radically increased the weight of the tank, the engine and transmission became overtaxed and resulted in significant maintenance issues.
Bottom line: the Sherman was a good design (and, in the end, a war-winning one), but one which had severe limitations when pressed into service for purposes which it wasn't designed for. Unfortunately, due to the lack of production of the M-26 Pershing and similar heavy Allied tanks, the Sherman frequently found itself being used in circumstances where its limitations were quickly exposed, usually to the serious detriment of its crew.