I definitely would be more comfortable with a party size of at least 5, strictly because it feels like the game needs to fill the roles of: Melee Fighter, Healer, Trap/Lock Wrangler, Ranged Attacker, Controller, and Multiple Target Damage. Now, 5e does have options to allow for flexibility, which allow for people to double up on some of those roles, but limits on class proficencies and ability score modifiers providing bonuses to skill proficencies can limit how useful that can be. Especially of note is how some skills are not represented at all in Backgrounds, and others are in many different backgrounds (although that issue can be somewhat fixed by allowing for backgrounds to customize your skill proficencies). Solasta just outright told the player in character creation that "X skill/language is not used in-game, so if you're taking that, it'll only be for flavor."
That said, Solasta also goes with a 4-person team and they have fewer options overall. I'm playing that game and trying out different party configurations to see how well I can cover roles with different parties and backgrounds. My first team was Barbarian/Cleric/Rogue/Sorcerer. My second team is Fighter/Druid/Ranger/Wizard. I'm sure there are more teams I can try and maybe the 4-person limit will feel fine in both Solasta and BG 3.
However, Solasta also is fairly strict with the rules on enemy HP, AC, saving throws, and options, and that can make a big difference.
