If you include all the players Howie drafted and signed as UDFAs since 2016 (before that it's hard to determine responsibility, he probably had a major role from 2011-2013, none in 2014 and 2015), he's got a pretty good record.
Teams generally get two starters and 1-2 solid reserves out of a typical draft, maybe a little better when you draft in the top ten or have extra picks. People exaggerate the value of the draft, go look at the rosters of playoff teams and they're a mix of draft picks, UDFAs, SFAs, a couple top FAs, some 2nd and 3rd tier FAs. One reason is many players don't hit their stride until their 4th or 5th year, so their original team doesn't want to pay the price of keeping them when they have a cheaper or better alternative like Vaitai - or like Hicks, they can't stay healthy.
While the GM makes the final roster decisions, they're basically determined by the evaluation of the coaches, coaches lobby for the players they want on the final 53. So when players leave due to waivers, it tends to be because they're not valued by their position coach. Fulgham for example probably got cut due to bad practice habits that alienated his coaches.