WWI had several undesirable features the most important being:
Tanks were not reliable: tanks broke down with astounding regularity at such a rate that initially most tanks would not make it through the first 24 hours of combat. The reliability was improved as experience was gained but remained high throughout the war.
Tanks were slow; about the same speed as a walking man or not much more (at least the large ones). The smaller variety achieved higher speed over firm and flat surfaces, such as they could encounter after a breakthrough. But to get there they had to navigate the crater infested battlefields, with additional obstacles such as trenches, rubble, etc. The operational speed was not impressive to say the least.
Explanation:
Tanks were toxic: men fought alongside a smoke generating engine, in a confined space in which they generated more fumes from their own weapons. Add the noise and heat of the weapons and engine and it is no surprise that crew performance suffered as a result.
Tanks were dangerous to the crew: they had no safety gear. As the tanks rumble across the battlefield, the crew was subjected to shocks and was liable to be thrown about, colliding with the mechanical gear and/or the weapons themselves. In a famous demonstration for the British King, half the crew was knocked out when they drove over an alleged bunker!
Tanks were not impervious to armor piercing rounds nor to field artillery: tanks sported large steel plates, but these were not in lined nor shape in any manner. Thus they relied on sheer depth to stop penetration. But penetration was not the only threat. The metal plates were subject to "spalding" where flecks of molten metal would detach from the inside surface of the plates, striking the crew and injuring them.
Tanks were relatively short ranged: the fuel consumption was high, the fuel reservoirs were small, thus severely limiting the combat range, not that they were expected to go that far: penetrating a few kilometers to the third trench system was all that was expected until plans were made for the 1919 Campaign.
They were heavy and cumbersome: once immobilized, they had to be repaired in situ. They were no tank recovery vehicles to speak of. This means that if and when the Germans recapture the battlefield in a counter-attack, they captured the tanks and could them back in operation for themselves. This happened more than what admitted at the time.
These are the 'cons'. What about the 'pros'?
If and where they did not break down or became disabled due to enemy fire or difficult terrain, they were very effective, especially in the absence of effective infantry anti-tank weapons. They did overcome the barbed wire, trenches, and most machine gun resistance encountered and facilitated if not created breakthrough for the following infantry.
Tanks were 'force multipliers', transporting a few men and weapons to the crucial battlefield spots where they could the most damage.
Tanks, at least initially, were shock and awe weapons. German infantry, generally defenseless against them (on their own), would be affected and this reduced their combat effectiveness. As experience was gained, they lost some, but not all, of the morale impact.
Tanks represented the weapons of the future. Given field experience and increasingly better technology, they were the precursor to the armored fighting vehicles of the 30's and 40's.
