Question about F3.5

I am thinking of building a 18in in F3.5. many guys
say it's too short, what type of problems do short focus flectors have, other then large secondary obstruction?
Thanks
Dennis

3 problems:

1) coma (distortion away from center of field) increases dramatically at very low f/ratios. It can be dealt with with a coma corrector, or can be mitigated somewhat by very expensive eyepieces -- in either case it basically takes money.

2) fussy collimation -- f-ratios this fast are pretty unforgiving of collimation/alignment errors, you need to be right on the money here to get real diffraction-limited performance.

3) Mirror quality / expense -- it is much harder to grind a good parabola at f/3.5 than, say, f/5-f/8 -- much deeper curve, lot more parabolic correcting to be done -- and thus, a lot more opportunity for error, or more effort required to achieve perfection.