The two main reasons I am very, very dissapointed about this party size reduction are:
Firstly It will weaken the ability of the party to control the battlefield by making it that much more difficult to prevent enemies infiltrating your lines. Not as much a Tyranny but obviously in that direction, and the factors that made Tyranny's combat lacklustre in comparisson to PoE will therfore adversly affect Deadfire.
If you cannot control the battle field (or mostly, even often, can't) then each character must per force be able to survive in direct contact with the enemy. This is Tyranny's "Stand & Deliver" problem: in the end every character must be built to withstand damage and simply dish out damage faster than they take it. You just stand where you can attack the most enemies at once and deliver as much damage as you can. There is little tactical interst and finesse in this. It gets boring and repetative quickly.
You also have the "Lantry problem". In Tyranny, certainly on PotD, in the early game when you recruit him enemies relentlessly target Lantry and he dies quickly and often. There is no reliable way to protect him tacticaly so the answer is give him the heaviest armour you can lay your hands on. It slows his casting but heavy armour makes him harder to kill and reduces his priority for the enemy AI. It works but it is boring and is not how I expect my RPGs to work.
Secondly it will severely limit options in party makeup. For me there are four mandatory positions in a party: Two melee specialists, a priest and a wizard. I know some people play replacing a priest with a druid and palladin, make tanky wizards and chanters, use cipher plus druid to cover wizard role etc, but I'm a boring conservative when it comes to this and I like to play traditional roles. I am not interested in multi-classing for example (although I know a lot of people are). I play PotD so two specialist melee (fighter/monk/palladic/barbarian) is a bare minimum and I always play with three in practice becasue I don't build tanky/melee casters.
In PoE this left two slots available for two of ranger, druid, chanter, rogue and cipher. Two out of five is not too bad. In Deadfire this is threatened to be reduced to one. I am very unhappy I will only be able to play one of those classes per playthrough (yes, I always take the same party through a game, I don't swap characters around).
What makes this worse is that my party has already been fixed in stone for my first Deafire game. We have Eder, Pallegina and Aloth comming with us and I have always played with them in my party anyway so they are mandatory picks. I am just finshing up my WM2 PotD run whichh will probably be my finakl Deafire save and I am a druid this time (thank God, otherwise unless I was a priest one of the main companions would have to go) so the only question is who is Durance's replacement is in Deadfire? That's literally all I have to look forward to in terms of new characters.
So why? With all these downsides to it and probably a lot more besides, what on earth are the upsides? I can't think of single upside, not one.