Specifically are same IP addresses with different masks
LAN:192.168.1.1/24 and WAN:192.168.1.1/26 problematic from routing and
NAT point of view and why ?
Yes, this is a problem, for routing in general.
Hosts on the LAN network will see the entire
/24 as a
CONNECTED route. Connected routes often have a special meaning, and can override more specific routes on remote networks. (what could get more specific than a host you have an ARP entry for?)
At a minimum, hosts on the LAN will be able to reach either the WAN subnet, or hosts on the LAN which are in the WAN range. (you could try adding a static route, for example.) But never both.
Also, WAN hosts will not be able to reach LAN hosts in the same IP range.
Some NATs (in particular, carrier-grade NAT) is capable of handling cases where the same subnet is on both the inner and outer side of the NAT. But obviously, this only works unidirectionally.
Are different IP addresses with same mask LAN:192.168.1.1/24 and
WAN:192.168.2.1/24 problematic and why ?
This is not, and has never been problematic. Lots of networks have the same mask, and it's not a problem. At the time of this writing, there are 260,864
/24 networks in the global BGP table. The only reason why your specific example could be problematic is because it's shared RFC 1918 address space. So if you imagine two organizations using 192.168.1.1/24, and one organization buys out the other organization, then there will be problems.