So this topic is about solo play rather than team play, whether in MP or SP? Because in team play (MP or SP) I would expect people to be choosing combos based the type of protections, not based on just their own character's powers.
Nope, not necessarily. Biotic death squad in MP will rofl stomp any faction without any sort of strategy except "spam BE." That includes Geth, who are shield heavy. It also largely doesn't matter which combination of biotics you use. Some are more overpowered than others, but they all come down to BE spam.
While i agree on that part, going for an exclusive power or weapon build (or anything inbetween, really) should definitely be viable, imo.
ME2's system was severely handicapping in that regard.
Yes, but clearly if you go gun only or power only you should not be as powerful as if you use an integrated approach.
With regard to ME2, despite all the flak it gets, it is much easier to go power only than weapon only, despite how many claimed powers were weak in that game. Of course some try to cite something like the Mattock (DLC gun...) under Adrenaline Rush (power) using Incendiary Ammo (power) as "weapon only," when it clearly is not.
This was not because of weapon multipliers though, this had to do with spare ammo on some weapons, the thermal clip drops, and the fact that it is harder to hit an enemy with a weapon than a power. With respect to the Mantis, it was underpowered in ME2 because of its strange spare ammo count. Off Hand Ammo Pack was also a poor armor mod that should have had a bigger bonus. That said, Mantis is still effective against mooks with headshots despite the armor multiplier. Soldier and Infiltrator both have a power to increase its damage to shields, and you can get extra headshot damage with the visor sold on Omega.
I do not begrudge them for putting a limit on clips in ME2 to encourage people to use powers on Insanity. It doesn't make sense to have an easy time going through the top difficulty ignoring half of the combat system.