Why is he stronger? The 2014 ranger gets no bonus on knowledge checks when in his favored terrains. He gets bonuses to knowledge checks ABOUT his favored terrains.
"When you make an Intelligence or Wisdom check related to your favored terrain, your proficiency bonus is doubled." Every knowledge check related to favored terrains is also a Nature check as that skill is for terrains, so the 5.5 ranger has expertise there as well.
And I'm not sure why you are bringing up languages. The 5.5e ranger gets languages as well. Maybe you mean the one bonus language for favored enemy, which really isn't a big deal when there are a lot of ways to communicate these days.
Is there something in the playtest that forces 5.5 rangers to use spells? Otherwise the rate of spell usage is entirely up to the player as it always has been.
Better than a 5.5 ranger with expertise in investigation and perception?
It gets the same amount of 1st, 2nd, 3rd, etc. spell slots as the 2014 ranger, so if it ran out quickly that was on you not the class. It does get two fewer cantrips, but those have unlimited uses still, so you still wouldn't run out of spells faster.
Until you quickly realize that you run out of spells that way and stop, sure. Self-control is something you might need when playing the 5.5 ranger. A player's lack of self-control isn't a weakness of the class, though.