Let me give you my full detailed analysis of the P-90 and Desert Eagle in Counter-Strike: Source.
The Desert Eagle or the Desert Nighthawk, mostly referred to as the Deagle, is the most powerful pistol in the Counter-Strike Series. Its .50 caliber rounds stops enemies nearly dead in their tracks. Its loud, booming and powerful signature sound makes players around the weapon a lot more cautious, especially if the player holding it is very adept to the weapon. From 2004-2011, the Deagle had a very high reputation for its 1 shot headshot deal. The drawbacks of this weapon however was the short 7 round clip it had. In addition to its high recoil which makes it a very bad choice of weapon of spraying from medium ranges. Even though it was designed for longer ranges than other pistols, the weapon also has long range capability. In late 2011, Valve decided to do a weapons patch which destroyed this once iconic weapon. Its damage ratio was brought down dramatically and its range also tweaked, not to mention its accuracy. Like most weapons in the patch, the damage is set by the range of your target by 'units'. But even though the weapon is what you call 'nerfed', it is still a very reliable weapon for those who still believe in the Deagle and can use it with love and respect.
The FN Project-90 (P90) is a dangerous weapon for up close engagements. Its 50-round magazine and high rate of fire makes this weapon the perfect choice for room to room and close range engagements. In fact, the P-90 has the highest ROF (Rate of Fire) in the Counter-Strike Series until the birth of the Negev in Counter-Strike: Global Offensive. The 5.7mm round the weapon uses is a unique round that has a high reputation in piercing through most objects (e.g wooden doors, thin steel doors, hollow boxes and the like) with ease. The recoil the P-90 possesses is easy to control in 5-10 shot bursts. In a 'spraydown', the weapon's recoil increases more and the spray pattern spreads, making the weapon ineffective at long-medium and long ranges. In 2004-2011, the weapon is most seen in public servers and sometimes deathmatches. But, it was not as often seen due to its relatively high cost. Which made other weapons like the Machine Pistol-5 Navy (MP-5) and the Schmidt Tactical Machine Pistol (TMP) be used more often. While the damage of the P-90 is relatively light, its higher ROF compensates for its damage to chew through opponents in close quarters. But likewise in Valve's weapon patch in Fall of 2011, the weapon underwent some major 'Modifications' in which most veterans call it, buffing. Now, the P-90 has relatively more effectiveness than the Model-4 series 1 (M4a1) Maverick Carbine and what not, more damage than it deserves. Its spray pattern in much more tighter and the accuracy rivals that to the FAMAS and Galil rifles. Thus. making this weapon much more of a spray and forget kind of person. As said by the majority of the vets, "This weapon takes no aim, very little in skill to kill anyone on the server they're in. Its ridiculous."