What you actually pay for it is irrelevant, if the tax is paid.
True, although charging less than the FMV makes it a taxable benefit which is more paperwork than simply charging the FMV.
So really you only save 15%.
It depends how the scheme is set up. As well as offering an option to defer until the FMV is lower, there’s no reason why the initial 12 monthly payments need to recover the full price of the bike.
I used to run an in-house scheme at my old employer, which also passed on the saving on the employer’s NI which employers typically pocket, and didn’t use vouchers so staff could buy discounted bikes from anywhere they liked. It can be a really good deal if you don’t get middleman voucher providers involved.