IMO, the most unique/best thing about the ER-101 is that it is extremely precise.
It is (without the 102) pretty linear. It runs through sequence and can loop them. You have a lot of flexibility around pitch (or cv), timing, and duration and you can dial in very specific relationships. You’re actually not that likely to get something you like by just turning and pushing (compared to something like Rene). It’s easy to fall off your grid and get a really sloppy mess (this is also its strength). If you have a specific idea in your mind, you can realize it very precisely and make super precise adjustments.
Caterina Barberi gets a lot of mileage out of it by making phased loops and adding to them piece-by-piece in ingenuous ways. (at least that’s what I think she’s doing). With the 102 things get a lot more interactive but you still need to do some prep-work before you get to the interaction part.
If you have facility with coding (or thinking in that way), teletype might be easier to “sketch” with, try out ideas, see if they result in something musical, adjust, etc. (But, that’s not me, and I don’t have a teletype so take my words with major grains of salt).