I have done a fair number of jazz duo gigs. I greatly prefer playing with a trio that includes some sort of percussion. I have done a lot of gigs where the drummer brought only a snare and brushes.
Drums generally play subdivisions with a relatively consistent pattern. The cymbals, high-hat, and snare tend produce a sharp transient that is easy to hear, so it's clear where the pulse is. IMHO, everyone is responsible for time, but drums is usually the primary reference. When drums are absent, the primary responsibility for time tends to fall to the bass, which IMHO is ill-suited for the task.
The big issue for me when there is no percussion is time tends to feel unsettled. I have a fairly bright and clear sound, but not a super strong attack. Without the strong attack, it's difficult for others to clearly hear where I am placing the beat, so the pocket never stabilizes. This makes it seem like the tempo and feel is constantly changing.
With just a guitar or piano, there is usually not enough of a structured rhythmic pattern to hang my hat on, so I pretty much have to focus on my internal clock and let the other player do whatever they are going to do. It's not easy to lay down consistent time when the other player's pulse is not maintaining a consistent relationship with my pulse.
So to me, the biggest issue with playing bass in a duo is it takes a lot more mental focus to maintain time. Also I have to resist the temptation to adjust to the fluctuating pocket, and this tends to make me feel anxious and uncomfortable.
If you put more of your focus on time, it takes away from your ability to interact and do things that may be more harmonically or melodically interesting. Bottom line is it's definitely harder in some respects, and you may even say it requires a bit of a specialized skillset.
IMHO, it's not only the bass player who must possess a specialized skillset. Playing bass in a duo is way easier if your partner has the necessary skills and experience to do it well. It's possible for someone to be a great ensemble player and lack the ability to play well in a duo. It's also possible for two skilled duo players to not really gel.
If you are playing in a duo and the other person lays out...you're playing solo. This requires yet another set of specialized skills. Personally, I think it's uncool when people automatically drop out over the bass solo, regardless of whether the group is a trio or duo (YMMV). But it's what many people seem to do unless you tell them otherwise.
