It is completely soloable, and of the 50 assaults there are only a couple that take long enough to allow tags to replenish all the way to maximum. Most can be done in well under 10 minutes, and that buffer will balance the rest a bit. Takes like 10 hours to do all 50 manually instead of ~8 to do them with ASE(10 min cooldown * 46), so it's not some immense time saver either unless you micromanage and step away(ingame or irl) every 5.
If I had to try to justify it, and I don't really like to, biggest complaint would be the obscure nature of many of them. If you haven't already done them 5x to know what to do, you have to look it up, read poorly written guides, and fumble through ineffectively. Having to zone 4 times per assault is also godawful, but ASE doesn't help at all there.
Orichalcum Survey is the only assault you can't guarantee a 1/1 win as a solo character with proper knowledge of the assault, and the odds still aren't that bad on it.