Nightstalker?

Two big issues with Night Stalker:

1) He tends to lose the lane to many common mids (it's rare for him to be killed or forced out, but he'll struggle for last hits/denies).

2) He needs a team to work around him, and for the enemy to be seperated if at all possible.

This tends to make him a tricky pickup in competitive matches - you don't want them enemy mid filling his boots, and organised teams are harder to get pick offs from. He's a classic snowballer who needs high farm/xp right from the start and needs to get kills to be really effective.

I'm a little surprised people don't think of him being PART of the deathball - during the early nights he's extremely scary and effective. If his team is backing him up they should easily be able to take towers and apply a lot of pressure during those times.

I think Nightstalker can be OKAY middle, but I don't think he's above average..

You're right to say that he plays very much like a Pudge. He will lose his lane and have to make it up with activity. The only problem is that he really can't kill the mid at all without assistance, where Pudge can hook into tower range and kill middle. Also Pudge doesn't have to put himself in a dangerous position to gank another lane, as he just sits behind the trees and pulls opponents out of lane. With NS you have to literally dive their lane, which makes you very vulnerable to TP support and getting the gank turned around.

I feel that there are heroes that do what NS does, but better for the most part. A Pudge pick for example... How many times has a Pudge landed a late game hook during a push that has completely turned the tide of the game? I suppose the NS equivalent would be a good silence, but I can't really recall seeing many.

I feel that NS is just too risky of a pick because of the necessary dive, and lack of a hard disable with his ganks. A lot of mids will pull opponents out of position to gank (Bat, Pudge), or if they have to dive the have a way back out (Qop, Puck). NS just dives and hopes he's tanky enough to survive the inevitable burst, disable, and tower shots. Seems riskier than most picks. Plus his teamfight is pretty garbage compared to a lot of mids.

I think the only time I drafted night stalker I had him in the offlane, dp mid and lycan safelane I had lycan running around killing anyone alone while the rest of the team wiped out every tower in 10 min. I think he fits well enough into some strats but not many and if the game isn't quickly won then its probably lost

IMO it's mostly been what the common picks or "Flavors of the Month" so far that is deterring his popularity. NS plays like any melee hero (i.e: Pudge), you try and get as much farm/exp as possible and gank or force rotations. So far popular mids have been heroes like Viper, OD, Razor; these heroes can literally deny both his EXP and farm if they play aggressively.

NS is a really snowbally hero compared to other heroes so if enemy heroes are able to play extra careful, NS will be forced to spend his time farming instead of ganking. If Nightstalker isn't able to create room, force rotations and have proper backup for dives, he'll fall behind REALLY hard.

Combined with the fact that it's so easy to predict where Nightstalker will go that pros will be prepared when he comes up. Nightstalker has one thing that he does really well, a strong ganker. If Nightstalker doesn't accomplish that, he'll be a bag of potatoes or a big creep since anything else he does, other heroes can do better.

Popular heroes also make a big impact yes, for example lifestealer used to be really popular, he still is really strong, but heroes like viper, razor and AA are really popular now and it just happens that he is countered by those.
Night stalker has the same problem, except he is even riskier.

I maintain an 80% winrate with Nightstalker by essentially ignoring mid once the first night comes around and clearing the lanes for my teammates. This seems to be a viable strategy in most pub games with no vision. However, once vision and competent players come into the equation, Nightstalker becomes obsolete.