I really like the rogue class. It is perhaps the most well written and round up that we have now.
Rogues have 3 subclasses. (5 with SCAG)
The sneak attack rule is clear. Either have advantage on the roll, have an ally beside your mark or both. Weapon must be a finesse one in order for the sneak attack to work.
Yes with the current rules, we see a lot of "ranged" rogue. I have two out of 4 roguish in my groups. So what? Nothing prevents them from going in hand to hand combat. Fog (and fog clould spells), heavy rains, strong winds and many other environmental effects (a dense forest?) are great to neutralize a ranged rogue. Use them. Make your players use their abilities to the maximum. And if you use the flanking rule, then the rogue will give advantage to the attack roll of his friend. This is a strong incentive for a rogue to get in thick of it.
For the assassin,
Yes some of the feature might seem a bit meh at first sight. The assassinate rule, in particuliar is countered by the alert feat. Now how many monsters do have that feat? None that I remember. So in a PvE environment, the assassination feature works out fine. It is only in a PvP that this might comes in. Or if the DM uses a lot of NPC built like the characters. Then I would almost think of it as cheating a player of his ability.
Anyways... In my group, the assassins do have to use this feature quite often. They are the scouts of their respective group. They get to sneak up on the enemy and use their assassination ability on small groups of enemy scouts, guards, patrols. If they get the catch on a lone beast/monster, that thing might be dead before the rest of the group comes in (this happened quite a few times, especially with Bretai the assassin.)
For the poison feature.
Assassin have always used poison. They will always use them whenever possible. I don't like poison users. But hey, if they get caught with poison on them they're doomed. Yet, poison has its place. With creative thinking, the player will come up with reasonable reasons to poison possession. "Look good sir, I hunt owlbears, I use poison on them because they're vicious, mean and evil beasts..." And it's not that easy to say something was poisoned with all the cuts and bruises it might have if the thing doesn't die immediately. So yes, the two assassins are using poison in key encounters or when they can get away with it. Rule is working as intended.
For the Infiltration Expertise.
That one depends largely on the skills of the player and the generosity or willingness of the DM. Bretai isn't that high, but Thomrax is using this feature like crazy. He used that feature to pass himself off as a Iuz emissary in a hobgoblin forteress, to act like a traveling merchant (at la Silk in the Belgariade), to a noble from the Great court in Rauxes in the Great Kingdom, a distant relative of Ivid V...
Maybe it's because my campaing is involving a lot of travel and it gives a lot of down time and travel time so that Thomrax can create a lot of different personalities. But in my book, it works out great.
For the Impostor.
Again, Thomrax is using that feature quite a lot. He is often in enemy territory to get intel on the forces of the Ancient. He is passing off as an obscure patrol captain of the boneheart that brings a lot of exotic goods from his "patrols" mainly spell components. Again the feature works as intended.
For the Death Strike.
None of my players are that high now. Yet, unless you build your NPCs as PCs are, none of them will have the Alert feat. That means that the feature should work as intended most of the time.
So far, the Assassins that I saw were quite good. Should I dare say almost too good? We have had quite a few arcane tricksters and thieves, but these didn't got as high as Thomrax as most of them either died an early death or had multiclasses in some way or another. Assassins, to my absolute dismay (I tend to prefer the arcane trickster as a DM), is the archetype that rose the highest level as a single class.