We've not seen much buyer's remorse in the 25 or so years that the Forum has existed. The TM is a niche product, appealing to a specific market, and owners are generally very pleased and loyal to their campers. Those who have owned pop-ups universally say how much of an improvement a TM is over a pop-up. All RVs require maintenance, and all experience occasional problems, but major issues are rare except in cases of severe neglect.
As for the shower walls, the TM comes with a two-part shower curtain, and it is important to use it to keep the walls dry. Some owners have invented "improved" shower curtains, usually with the goal of increasing the space in the shower - but one way or another, the first goal is to keep the walls dry. I don't think I've ever heard of a shower wall problem.
At 13,500 or 15,000 BTU, the roof-mounted AC is more than adequate for the interior space of a TM. Early models had an AC mounted in a cabinet near the kitchen floor, which some considered marginal (but some did not). The only common comment is that it is loud - but I think that is true for most non-ducted units.
Keep the questions coming. It is what we do here.
Bill