I don’t think it’s the short eye relief on the crossbow scopes - but rather it takes away one inch of eye relief.The short eye relief on the crossbow scopes seems to cause problems with this attachment.
I would assume if you used a 1” forward extended scope mount, then the 1” the FTS takes away would be the same as without the FTS.
I could move the forward mounts up an inch on 2 of my crossbows to use the FTS, but then I’m hostage to having to use it even for target practice because the scope is an 1” too far away.
I really wanted to get an FTS but not sure I’d like it now. I have enough crap on my crossbows as it it!