Rogue gets ASI at 8th level vs as 1st level fighter:
Fighting style(wotc values this as full feats, but most will barely be enough for a half feat if that. but that is another topic)
medium armor proficiency+shields, that is half feat.
martial weapon proficiency, that is half feat.
second wind, that could be a half feat.
so in total(I will rank fighting style as half feat as it in not worth full feat. not even archery): that is 2 feats(more or less) vs rogues 8th level ASI.
And that same character has to wait longer for every rogue class feature after that. One level later the pure rogue adds sneak attack damage and evasion that the splash doesn't have. The first epic boon is lost completely.
Medium armor does little for a rogue because they are typically dex based anyway. The shield takes away the off hand attack to cut down the chance of landing sneak attacks.
Martial weapons don't matter because of the sneak attack needs to be done with a ranged or finesse weapon. Rogues already have proficiency in all martial weapons that have the finesse quality. The fighter splash adds and average of 1 damage on the die before calculating hit ratios by using a heavy crossbow at range, and using nets.
They're getting second wind and a fighting style instead of a feat. The shield is a damage trade-off but the feat could be defensive duelist anyway, and everything else doesn't help. I'm not really seeing the issue in your example.