The biggest downsides to companions is that the A.I. is generally just retarded and boneheaded. They cannot traverse hills or mountains without getting completely lost. They often freeze in combat or engage in the most inefficient ways possible. They aren't programmed to use certain healing or aid items. It just kills the immersion when they are running headfirst into a Deathclaw group. Or running around in circles in one spot and shooting their weapon into a rock formation like a chicken with its head cut off.
The companions also barely interact with each other and barely interact with the world either. Obviously this would take tons of programming and work and time to seamlessly involve the companions with the entire game world like the player is in something like New Vegas. But it does hurt the immersion when the companions have almost zero autonomy.
Companions have gotten a complete overhaul in a Fallout adjacent game, Outer Worlds(also made by Obsidian). In that game, the companions seem to be a lot smarter, more effective, have special moves they can use, you can actually improve how much more powerful they get by handing them out perks every few levels and leveling up Leadership skills for more health/higher damage dealt. They also interact a whole lot more with the world, both with each other and NPCs, as well as comment on almost every location, that includes DLCs. I would love to see a Fallout game with companions done in this fashion, I think how they were handled in New Vegas was a prototype(or an inspiration) for how they would be done in OW nearly 10 years later. That game aside, I do like how Bethesda done companions and made them an integral part of the game in 4, altho they are still braindead in combat and don't have too much interactions outside main quests and their own quests.
Still, we've come a long way since the 3 hanger-ons in 1. Funny story with them, companions were not even going to be in the game until the last minute, that's why they are so basic(you cannot even give them any armor in 1 without fan fixes and they can only use very limited weapon pool due to their sprites only using some weapon animations). If I recall companions were only thought of when a NPC duplication glitch effected Ian, one of the NPCs that was supposed to help you with radscorpions in Shady Sands. Having several Ians help you out when the glitch happened was so fun that the devs decided it would be cool to have more NPCs follow you and fight alongside you anywhere you went, you can tell they wanted to make that a proper game mechanic in 2 right from the get-go. Even funnier that Fallout 3 would repeat history, companions in that game were also done at the last minute since Bethesda didn't have experience with them in their own games, and they infamously didn't even have time to program in radiation immune companions to walk into the chamber at the end in your place(fixed with DLC but still). Without mods, most of the time the companions in 3 will not even equip their weapons, they will intensely stare at the enemy with their arms on their side and get shot at unless you talk to them during combat and enter their inventory so they can "equip" all their inventory again. Pray to your lucky stars that they fixed this in NV, otherwise companions would be unusable in that game as well.
