In fact, since the checkpoints for all lineages are often the same, the inner core of all practices is often identical.
At a basic level, it's possible to empty the mind for 3-4 hours. Same with Wang Liping, vipassana, or Mo Pai.
The only difference is in some external movements and rituals. And most of those gurus who advocate not mixing different lineages are actually concerned with monetizing spirituality and their bank accounts.