Most hitch extensions I’ve seen reduce tongue
weight rating by 50% (see the fine print). That, combined with the extra weight of the rack and bicycles, means you need a pretty robust vehicle to start with.
Let’s say you have a mid-sized crossover with a 500# tongue weight rating. The extension drops that to 250#. The trailer uses up 200#, leaving only 50# for rack and bicycles.
Hitch extensions also move the ball away from the vehicle, which is bad for towing stability (sway), though not as serious as low tongue weight.
Last, bicycles on a hitch extension block access to the cargo area of the vehicle. Maybe that’s good, because with your tongue weight rating maxed out, you won’t want to carry cargo anyway...
No simple, easy solutions for bicycles, unfortunately. That’s why there are so many threads on the subject!