Any disadvantages for no companions?

Once you've removed their utility, companions are good for being travel companions.

1) Companions have random dialogue triggers. I was with Nick in Diamond city, and he and a random resident had a conversation about a missing person. On the Prydwen, he had a few snarky comebacks to the Brotherhood scribes on his condition. CVRIE humorously misinterprets Railroad slang. In many major quests, companions have slots for dialogue when you're talking to an NPC.

2) Companions provide firepower. Have you ever wanted to storm into a building with two gatling lasers? Hand one to a companion that can use weapons.

3) Companions give you presents. Dogmeat will bring you items. CVRIE will craft stimpacks for you.

4) Romantic companions give you a large XP boost. Instead of well rested (+10% XP, allied beds only), you get the Lover's Embrace perk (+15% XP, any bed).

5) Humanoid companions can transport power armor. This sometimes gets dicey with quest dialogue lockouts, but companions make good mobile armor stands.

You'll miss the companion quests, which can be interesting. Each companion has a quest that you can't access otherwise.

You're also missing the companion perks, each one has a unique perk that you can only get by getting your companions to idolize you.