my personal experiences with Mei are:
1) Takes a hit
2) You can't take a hit from her
3) You can't heal (ignite)
4) You can't use skills (acupuncture + immobile)
5) Mei team faster than you, you lose...
Personally, I can sometimes beat Mei, but she is a Super Rare tier ninja.
Tendo, Chikamatsu, Hashirama, Ay, are all easier to deal with, due to different ways of countering them.
I won't say that Mei IS unkillable, she definitelly is possible to counter, but the problem is... She counters more things than there are things that counter her.
The only 2 ways I found to beat her, while being slower on initiative are:
1) Poison/Ignite her to death (Poison Tai, or various sources of ignition)
2) Be lightning Main and play Sth like Asuma WB, Iruka, Sasuke, to just kill one ninja with standards from lght main.
The whole "she is squishy" applies only to the 2 situations listed above, or 100% blitz teams.
Tank and balance teams cannot do anything, while facing this monster. I won't say she is Kimimaro level tank, but she is definitely not Kakashi level of squishiness.
I won't say she needs to be nerfed, but at the same time I can't agree with the choices made while making a talentset for this character.
Another argument I see here, her damage is low. Her damage takes away half of my whole team's health, it's as high, if not higher than GNW1010 most of the time, + it ignites the whole team.
Her not chasing her mystery is the only argument valid, that I agree makes dealing with her remotely possible, but still is not enough to compensate for all other advantages she has.