PVP pyramids meant for battle are typically (well, hopefully) constructed and removed the same day. Basically, a FOB. Constructing one next to an enemy base and leaving it there forever and ever, I could see the issue.

Heat traps don’t require that many fireplaces really. Small enough space you only need two or 3. The question there is the actual use of heat (or any temperature/environment change) as a weapon being an “exploit of game mechanics” as some would suggest.