Why tcl
can be a problem:
You can bypass triggers by using a different way or not using switches/doors. AFAIK you will not trigger triggerboxes during tcl even if you fly through them.
That leads to skipped quest stages, which can lead to borked savegames.
You can also enter areas (say Cabareth) were you are not supposed to be. That triggers quest stages to early /without the prerequisites which also leads to borked savegames.
Why unlock
can be a problem:
Some doors and areas behind doors trigger quest stages.
Triggering them without the prerequisites leads to borked savegames.
Say the Soulflayer Temple which is part if the (late) mainquest.
You can use tcl and unlock, but you have to exercise some caution, especially in the main quest and when accessing inaccessible areas or "key required" doors.
Like
this dude should have done.
