Equity offer in a startup trading firm

Got an offer from a new HFT market maker startup for a quant position, unusual thing is that they give equity as a part of the offer. It’s roughly 50% cash base and 50% equity, they also said that discretionary bonuses in the first years will primarily be paid in equity rather than cash.

The offer is good otherwise (total comp matches top firms but >50% of it is paper money with unclear value) and founders are reasonable (experienced from top market maker firm).

How should I think about it? I never thought about equity in prop trading, always thought about it as a tech thing where you wait for the firm to get hyped and cash out on IPO. Prop shop will likely never IPO. How much can I expect shares to appreciate in value? Looking at public trading firms like Virtu and Flow, they are not going up after IPO. 
Is it ever a good deal to have equity in a prop trading firm, compared to let’s say, % profit sharing?

Looks like equity is very illiquid and I guess shares will grow in price slower compared to revenue. Am I wrong?