I don’t think anyone ever underrated Sampras’ forehand. At his peak, it was the best on tour even over Courier’s power, Chang’s pass and Andre’s return ( I know, very bold statement!).
Off that side he hit his contemporaries’ best shots about as well as they did. This isn’t to say he was as capable a backcourter as they were though. His backhand, which went from dodgy to solid and back again quite often throughout his career, prevented him from being a complete baseline campaigner. But if it were up to one side, Sampras was the best for two big reasons: the defense-to-offense forehand and the transition forehand.
As many people know, Pete protected his backhand side by camping a foot or so to the left of the center mark. This meant his opponent had 1/4 of the court to get to his backhand, a shot that he perfected-the-art-of-neutralizing-his-opponent’s-offense with. And out of desperation, foolishness or pure boldness, people would hit to the open forehand corner and into one of the single most devastating shots at the time – his running forehand. It was better than Courier’s and Agassi’s ( who was always in trouble on step number three ), more penetrating than Chang’s and just a bit more of an angle, variation and reach than Lendl’s – but only by a pinch.
And he could do just about anything with it, with big powerful blast to either corner. When he could really move ( “93 to “97 ) it would almost always be the outright winner or it would get him the weak mid-court ball.
In addition, at his best before age and injury made him rely more on his serve ( much like how Jordan relied more on his fade-away jumper towards the end of his career ) Sampras’ forehand allowed him to more forward better than anyone. That’s why he was a better offensive baseliner – he can transition to the net with greater power and accuracy than the rest of the tour. And it was that ability separated him from everyone else. All things considered, if it was a hardcourt I’d stack his ground game up against anyone’s.
Now of course defensively is a different story. Again, I blame his backhand; Pete on his back foot moving to his left is not the same as moving to his right.
And in regards to Federer, their forehands are quite similar especially on the run and inside out. The one major difference between their forehands is Roger’s ability to hit the shot from his backhand court ( left of the center mark ) to his opponent’s forehand court. Roger’ mechanics allow him to hit a more consistent and penetrating shot, with a higher net, a shorter court and a ball height beneath the net. If it was high above the net that’s a different story but a low ball from there was always a bit of a crap shoot for Pete. Something like that is routine for Roger.
Ultimately, although he made life difficult for himself with some tough shots and stroke production which could sometimes be described as "complex", at its best Sampras’ forehand was one the premiere shots in tennis.