LTE-U will first seek out channels with no incumbent Wi-Fi (or LTE-U) access points. If no clear channel is found, it'll seek the least occupied one, then evaluate how many other transmitters are using it. It'll then use the channel for a an amount of time proportional to that total number of transmitters. So if sharing with 2 other Wi-Fi access points, it'll take one third of the time. The result is that the impact of LTE-U on Wi-Fi is no more than the impact of Wi-Fi on Wi-Fi.
I would argue that the impact of LTE-U on Wi-Fi is actually greater than that of Wi-Fi on Wi-Fi, despite what you have pointed out. It is true that LTE-U operates on a fair time-share basis (TDMA [1]), however the problem here is that LTE-U unfairly transmits while Wi-Fi stations are transmitting, giving absolutely no regard to the actual channel usage by Wi-Fi stations. Wi-Fi does not work on a time-share basis, it operates on a "don't talk while I'm talking" basis, and since TDMA is a different way of communicating, Wi-Fi stations don't know how to avoid collisions with LTE-U stations. From the perspective of a Wi-Fi station, an LTE-U station is just like a leaky microwave oven, there is no way to avoid radio collisions with it.
There isn't a way for Wi-Fi stations to know what TDMA time slots are "free," how long those time slots are, and there isn't a way for Wi-Fi networks to "sign up" for a time slot with the TDMA networks. The TDMA networks just decide to talk at some predefined interval without consulting the Wi-Fi networks to see when they would like to talk. This does not sound fair to me - it's kind of like saying "I'm going to yell very loudly once every minute. If it interrupts the conversation that you are trying to have then too bad for you. But I'm not going to yell when my friends are yelling, because we have an agreement in place."
Normally, Wi-Fi stations use CSMA/CA [2] to avoid transmitting when other Wi-Fi stations are transmitting, which overall reduces the number of lost Wi-Fi frames for everybody. This is what I mean by "don't talk while I'm talking." So the impact of Wi-Fi on Wi-Fi is materially lower than the impact of LTE-U on Wi-Fi simply from a statistical standpoint.
In addition, LTE-U only transmits when it detects other Wi-Fi transmitters. However, due to the hidden node problem [3], not all Wi-Fi stations that are using the channel can necessarily be detected by an LTE-U station, but the LTE-U station is still able to interfere with them. So there will be some quiet Wi-Fi stations that the LTE-U station can't see at all, and when the LTE-U station decides the channel is mostly empty and blasts its signal there, the quiet Wi-Fi station will be disproportionately overcome with interference. Again, the interference would be TDMA interference, but the Wi-Fi access points would not know how or when to avoid TDMA collisions, so the interference would be worse than Wi-Fi interference.
Normally, with Wi-Fi and CSMA/CA, the gaps in the louder station's transmissions would be good opportunities for the quieter station to start transmitting, allowing them to sometimes take turns even though the louder station cannot hear the quieter station, and the hidden node problem would be mitigated as best as CSMA/CA is capable of doing so. But with TDMA interference, there isn't any cue that Wi-Fi can take which indicates that now is a good time to transmit without interference.
This could all be avoided if LTE-U didn't solely operate on a TDMA basis, but also ceased transmitting (or skipped time slots) while Wi-Fi networks are transmitting.
LTE-U risks starting or worsening a "loudness war" where other unlicensed spectrum users decide to buy louder and louder unlicensed equipment up to (and possibly past) the legal limit, because their networks are unusable with too much interference otherwise.
Combine this design decision with the interest that wireless telecommunications companies have preferring their LTE networks over the well-being of Wi-Fi networks, and it starts to paint a sinister picture - that telecommunications companies might be attempting to deliberately interfere with unlicensed Wi-Fi networks in order to promote their own networks and equipment as being superior.