Is Yuumi weak alone?

She's got to be one of (if not the) worst solo laner in the game, currently.

Her abilities were designed for her to be attached to someone, taking that away leaves you with a squishy, immobile champion with no wave clear. She's a sitting duck in any lane but bot.

.... Yes? Her abilities literally require another champion to be effective.

Her entire playstyle revolves around being combined with someone.

Yes and No. She has an on hit Shield every once in awhile as well as a semi decent line of sight poke, and when used on self, her heal is actually more viable than on an ally.

The question really is..... can she KILL someone in a solo lane. The realistic answer is, probably not, unless being built as an ADC, which requires considerable skill.

If you want to just be an annoying second support, it's viable, especially if the main support is a tank like Thresh.

She has no damage from her passive. Her Base AD is 55 +3 per level, which is average for a ranged mage. Her base armor is 25, which is low-average for ranged champions. Armor growth is +3 per level, tied for 5th lowest in the game. Her base magic resistance is 25 and her MR growth is +0.3, which are both the lowest in the game. Her Q is a painfully slow, linear, single target, skillshot with low damage if you can't get the empowered damage, which is really hard to do when not attached to an ally. W is pretty useless by yourself. Basically just a little bit of extra AP E is a good heal and movement speed boost. Probably her best non-ultimate ability when solo. Her ultimate is GREAT for setting up ganks and peel. .. but the damage is still pathetic.

That said, .. if your allies just want to babysit you 24/7, you can put a decent amount of pressure on that lane.

I made a suggestion post to give Yuumi a buff that only works when there are no allies within something like 2500 units that boosts her damage by a percentage of the target champion (maybe monsters) current HP. This would make her more of a threat to high health champions who have her cornered, but it basically works to balance the threat more than giving her kill pressure.

Of course. She has jack shit in her kit to function in any role but support. She has no waveclear to do anything mid, is too squishy to exist top, and has no ability to clear, gank, or even survive fighting buff to jungle. ADC would theoretically work except you want to spend more time attached and she doesn't really have anything that makes her a better ADC than Ahri, except for maybe the effective Zhonya's on her W to let her hide on her tank until the assassins go away.

Some cheese strat may come up eventually to allow her to occasionally function in a solo lane, but that's it unless she gets some mechanical change (such as AoE on Q).

What? you mean a champ with a slow moving skill shot as her only for of damage outside her ult is weak in laning? She can totally fight off against a He only had 3 damaging abilities in his kit besides his ult. It is a completely fair and balanced lane, you must be playing it wrong.