The Wiegand interface is not bi-directional and supports no authentication.
If it is possible to gain physical access to the wiegand communication wires, then an attacker can record a valid registration and replicate it at any later moment to grant themselves access.
The Wiegand 26 bit format is very short and only supports a limited number of unique numbers.
The license plates will be turned into a number within the supported id-number range using the SHA1 hashing algorithm, but that doesn't increase the number of unique id-numbers.
Only 65535 unique id-numbers and 255 facility-codes are possible within the Wiegand 26-bit range. This can result into a maximum of 16 million unique combinations of facility-codes and id-numbers.
Because there are many more license plates the is a reasonable chance that 2 license plates will result in the same id-number.
Below a few license plates that all give the exact same Wiegand 26-bit SHA1 facility-code 102 and id-number 9890:
VR46IT, 67CJ76, IP47JV, KN91RW, ZH88FD, KKFC31, NCMU77, PLIX58, WLZQ03, 52RXLR, 56PQSE, 67NRQV, 95XNEQ, 09IRC6, 88IWB8, 5RVZ96, A173RQ, KDF99Y, RQE86P, UAG78D, Y82XEF, Z20PRO, Z66HQK, Z95NOO, 2NR367,
Note that there are many more duplicates.