If they are valuing the common stock based on the last funding round which sold preferred shares, you are likely significantly overpaying both for your exercise price and in AMT tax, and that makes the investment significantly riskier.
If common share FMV is discounted appropriately, IMO a 409a valuation will reflect the extreme risk of common shares dilution in an unprofitable venture with large investor preferences, and you should have a very low exercise price with little to no AMT unless the company is already well into planning an IPO.
At past companies I saw common stock valuation that matched the funding round valuation, and you should never be paying that price for common shares. 1/10th that price is a good rule of thumb.
The second most important factor is that you can’t ever sell shares in a company that doesn’t have a market for its shares. That vast majority of companies do not have, and never will have, marketable securities.
The third factor for me would be if the shares are eligible for QSBS Section 1202 treatment (tax free up to $10 million), which ISO options are not, but NSOs can be but the 5 year holding period starts at the exercise date.